Expected Behavior

When building documentation (dune build @doc) for a project containing private libraries I expect the documentation to not show the private libs.

Actual Behavior

The documentation shows private libraries:

$ dune build @doc
$ cat _build/default/_doc/_mlds/foo/index.mld 
{0 foo index}
{1 Library foo.bar}
The entry point of this library is the module:
{!module-Foo}.
{1 Library secret}
The entry point of this library is the module:
{!module-Secret}.

Reproduction

1. Set up private library following

   dune/test/blackbox-tests/test-cases/private-package-lib.t/run.t

   Line 38 in d75bd88
   until Line 38
2. Build docs dune build @doc
3. The documentation shows the private library cat _build/default/_doc/_mlds/foo/index.mld

Specifications

• Version of dune (output of dune --version): 2.9.0, but 2.8x behaves the same
• Version of ocaml (output of ocamlc --version) 4.12.0
• Operating system (distribution and version): openSUSE Tumbleweed

Additional information

• Link to gist with verbose output (run dune with the --verbose flag): https://gist.github.com/tjammer/a97bb147c51e0033c4b6f4418435f6b9