"Rule failed to generate the following targets" with virtual package

I am working on the JavaScript support for a package called [bls12-381](https://gitlab.com/dannywillems/ocaml-bls12-381/) implementing the curve BLS12-381 using a Rust optimized implementation.
The initial version of the package `bls12-381` was only for UNIX, and was using an intermediate Rust dependency called [rustc-bls12-381](https://gitlab.com/dannywillems/rustc-bls12-381).
The JavaScript part uses jsoo to call a wasm version of the rust code given above.

I decided to go with the following structure:
- bls12-381 -> virtual package. Code located under `src/virtual`
- bls12-381-unix -> UNIX version, implementing bls12-381, using the rust code compiled into a static library. Code located under `src/unix`
- bls12-381-js -> JavaScript version, implementing bls12-381, using jsoo to wrap a NPM package containing the wasm code and providing a JavaScript layer above this wasm code. Code located under `src/js`, mostly calling `bls12-381-js-gen` under `src/js-gen`.

Intermediate packages are used for modularity/to ease the implementation of UNIX/JS support (bls12-381-gen, bls12-381-js-gen). So the structure is more:
```
- bls12-381-gen
  -> bls12-381
  |  --> bls12-381-js-gen
  |  | --> bls12-381-js
  | --> bls12-381-unix
```
When building and running the tests with `dune build @runtest`, everything is fine. However, when I try to install with opam (see below), I get a bunch of the following error for different files:
```
# Error: Rule failed to generate the following targets:
# - src/unix/.bls12_381_unix.objs/native/bls12_381.cmx
```


## Expected Behavior

Compile without errors.

## Actual Behavior

```
$ opam install bls12-381-unix

<><> Synchronising pinned packages ><><><><><><><><><><><><><><><><><><><><><><>
[bls12-381-unix.dev] no changes from file:///home/danny/codes/dannywillems/ocaml-bls12-381

The following actions will be performed:
  ∗ install bls12-381-unix dev*
[bls12-381-unix.dev] synchronised from file:///home/danny/codes/dannywillems/ocaml-bls12-381

<><> Processing actions <><><><><><><><><><><><><><><><><><><><><><><><><><><><>
[ERROR] The compilation of bls12-381-unix failed at "/home/danny/.opam/opam-init/hooks/sandbox.sh build dune build -j 3 -p bls12-381-unix @install".

#=== ERROR while compiling bls12-381-unix.dev =================================#
# context     2.0.6 | linux/x86_64 | ocaml-base-compiler.4.09.1 | pinned(file:///home/danny/codes/dannywillems/ocaml-bls12-381)
# path        ~/codes/dannywillems/ocaml-bls12-381/_opam/.opam-switch/build/bls12-381-unix.dev
# command     ~/.opam/opam-init/hooks/sandbox.sh build dune build -j 3 -p bls12-381-unix @install
# exit-code   1
# env-file    ~/.opam/log/bls12-381-unix-1063079-d2c37b.env
# output-file ~/.opam/log/bls12-381-unix-1063079-d2c37b.out
### output ###
# [...]
# File "src/dune_rules/virtual_rules.ml", line 40, characters 30-37:
# Error: Rule failed to generate the following targets:
# - src/unix/.bls12_381_unix.objs/byte/bls12_381__G2.cmi
# File "src/dune_rules/virtual_rules.ml", line 40, characters 30-37:
# Error: Rule failed to generate the following targets:
# - src/unix/.bls12_381_unix.objs/byte/bls12_381__Pairing.cmi
# File "src/dune_rules/virtual_rules.ml", line 40, characters 30-37:
# Error: Rule failed to generate the following targets:
# - src/unix/.bls12_381_unix.objs/native/bls12_381.cmx
# File "src/dune_rules/virtual_rules.ml", line 40, characters 30-37:
# Error: Rule failed to generate the following targets:
# - src/unix/.bls12_381_unix.objs/native/bls12_381.o



<><> Error report <><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><>
┌─ The following actions failed
│ λ build bls12-381-unix dev
└─ 
╶─ No changes have been performed
```

```
$ opam install bls12-381-js

<><> Synchronising pinned packages ><><><><><><><><><><><><><><><><><><><><><><>
[bls12-381-js.dev] synchronised from file:///home/danny/codes/dannywillems/ocaml-bls12-381
[bls12-381-js] Installing new package description from upstream file:///home/danny/codes/dannywillems/ocaml-bls12-381

The following actions will be performed:
  ∗ install bls12-381-js-gen dev* [required by bls12-381-js]
  ∗ install bls12-381-js     dev*
===== ∗ 2 =====
Do you want to continue? [Y/n] Y
[bls12-381-js.dev] synchronised from file:///home/danny/codes/dannywillems/ocaml-bls12-381
[bls12-381-js-gen.dev] synchronised from file:///home/danny/codes/dannywillems/ocaml-bls12-381

<><> Processing actions <><><><><><><><><><><><><><><><><><><><><><><><><><><><>
∗ installed bls12-381-js-gen.dev
[ERROR] The compilation of bls12-381-js failed at "/home/danny/.opam/opam-init/hooks/sandbox.sh build dune build -j 3 -p bls12-381-js @install".

#=== ERROR while compiling bls12-381-js.dev ===================================#
# context     2.0.6 | linux/x86_64 | ocaml-base-compiler.4.09.1 | pinned(file:///home/danny/codes/dannywillems/ocaml-bls12-381)
# path        ~/codes/dannywillems/ocaml-bls12-381/_opam/.opam-switch/build/bls12-381-js.dev
# command     ~/.opam/opam-init/hooks/sandbox.sh build dune build -j 3 -p bls12-381-js @install
# exit-code   1
# env-file    ~/.opam/log/bls12-381-js-1063579-038aac.env
# output-file ~/.opam/log/bls12-381-js-1063579-038aac.out
### output ###
# [...]
# File "src/dune_rules/virtual_rules.ml", line 40, characters 30-37:
# Error: Rule failed to generate the following targets:
# - src/js/.bls12_381_js.objs/byte/bls12_381__Fr.cmi
# File "src/dune_rules/virtual_rules.ml", line 40, characters 30-37:
# Error: Rule failed to generate the following targets:
# - src/js/.bls12_381_js.objs/byte/bls12_381__G1.cmi
# File "src/dune_rules/virtual_rules.ml", line 40, characters 30-37:
# Error: Rule failed to generate the following targets:
# - src/js/.bls12_381_js.objs/byte/bls12_381__G2.cmi
# File "src/dune_rules/virtual_rules.ml", line 40, characters 30-37:
# Error: Rule failed to generate the following targets:
# - src/js/.bls12_381_js.objs/byte/bls12_381__Pairing.cmi
```

## Reproduction





1. `git clone https://gitlab.com/dannywillems/ocaml-bls12-381 && cd ocaml-bls12-381`
2. `opam switch create . 4.09.1 --no-install`
2b. `git checkout remove-pin-depends` # temporary, require a MR to be merged in ocaml/opam-repository
2c. `opam pin add tezos-rust-libs.1.1 "git+https://gitlab.com/tezos/tezos-rust-libs.git#f0a269087fe86dbe1cf471bb76e42fb4a481e226"` # temporary, like 2b.
3. `opam pin add bls12-381-gen.dev ./ --no-action`
4. `opam pin add bls12-381.dev . --no-action`
5. `opam pin add bls12-381-unix.dev . --no-action`
6. `opam pin add bls12-381-js-gen . --no-action`
7. `opam pin add bls12-381-js . --no-action`
8. `opam install bls12-381-gen --with-test`
9. `opam install bls12-381 --with-test`
10. `opam instal bls12-381-unix --with-test` # will fail with the error above.
11. `opam instal bls12-381-js-gen --with-test` # fine.
12. `opam instal bls12-381-js --with-test` # will fail, with the error above.

However, when using `dune build @runtest`, it does work fine.


## Specifications

- Version of `dune` (output of `dune --version`): `2.8.2`
- Version of `ocaml` (output of `ocamlc --version`): `4.09.1`
- Operating system (distribution and version): ubuntu 20.04
- Version of `opam`: `2.06`.

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"Rule failed to generate the following targets" with virtual package #4195

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