$ cat Foo/Project.toml
name = "Foo"
uuid = "9b094d01-e697-4a76-8fe8-871656a862e0"
authors = ["Cody Tapscott <topolarity@tapscott.me>"]
version = "0.1.0"

[deps]
Bar = "ed96e9dd-c367-45ab-9d59-befe4c57f37f"

[weakdeps]
Baz = "5796e38b-8bd2-4f06-8c98-e0a8826bb787"

[extensions]
FooBarExt = "Bar"

$ ./julia --project=testenv -q
julia> using Foo, Bar
Info Given FooBarExt was explicitly requested, output will be shown live
ERROR: LoadError: ArgumentError: Package FooBarExt does not have Bar in its dependencies:
- Note that the following manifests in the load path were resolved with a potentially
  different DEV version of the current version, which may be the cause of the error.
  Try to re-resolve them in the current version, or consider deleting them if that fails:
    /home/topolarity/repos/julia/testenv/Manifest.toml
    /home/topolarity/.julia/environments/v1.12/Manifest.toml
- You may have a partially installed environment. Try `Pkg.instantiate()`
  to ensure all packages in the environment are installed.
- Or, if you have FooBarExt checked out for development and have
  added Bar as a dependency but haven't updated your primary
  environment's manifest file, try `Pkg.resolve()`.
- Otherwise you may need to report an issue with FooBarExt
Stacktrace:
  [1] macro expansion
    @ ./loading.jl:2262 [inlined]
  [2] macro expansion
    @ ./lock.jl:287 [inlined]
  [3] __require(into::Module, mod::Symbol)
    @ Base ./loading.jl:2234
  [4] #invoke_in_world#2
    @ ./essentials.jl:1082 [inlined]
  [5] invoke_in_world
    @ ./essentials.jl:1079 [inlined]
  [6] require(into::Module, mod::Symbol)
    @ Base ./loading.jl:2227
  [7] include
    @ ./Base.jl:582 [inlined]
  [8] include_package_for_output(pkg::Base.PkgId, input::String, depot_path::Vector{String}, dl_load_path::Vector{String}, load_path::Vector{String}, concrete_deps::Vector{Pair{Base.PkgId, UInt128}}, source::Nothing)
    @ Base ./loading.jl:2825
  [9] top-level scope
    @ stdin:5
 [10] eval
    @ ./boot.jl:439 [inlined]
 [11] include_string(mapexpr::typeof(identity), mod::Module, code::String, filename::String)
    @ Base ./loading.jl:2678
 [12] include_string
    @ ./loading.jl:2688 [inlined]
 [13] exec_options(opts::Base.JLOptions)
    @ Base ./client.jl:327
 [14] _start()
    @ Base ./client.jl:558
in expression starting at /home/topolarity/repos/julia/Foo/ext/FooBarExt.jl:1
in expression starting at stdin:5
  ✗ Foo → FooBarExt
Precompiling FooBarExt finished.
  0 dependencies successfully precompiled in 1 seconds. 2 already precompiled.

┌ Error: Error during loading of extension FooBarExt of Foo, use `Base.retry_load_extensions()` to retry.
│   exception =
│    1-element ExceptionStack:
│    The following 1 direct dependency failed to precompile:
│
│    FooBarExt
│
│    Failed to precompile FooBarExt [1cbedc19-d9e5-5698-ba6d-aa629f4ffcbf] to "/home/topolarity/.julia/compiled/v1.12/FooBarExt/jl_UIHQPh".
│    ERROR: LoadError: ArgumentError: Package FooBarExt does not have Bar in its dependencies:
│    - Note that the following manifests in the load path were resolved with a potentially
│      different DEV version of the current version, which may be the cause of the error.
│      Try to re-resolve them in the current version, or consider deleting them if that fails:
│        /home/topolarity/repos/julia/testenv/Manifest.toml
│        /home/topolarity/.julia/environments/v1.12/Manifest.toml
│    - You may have a partially installed environment. Try `Pkg.instantiate()`
│      to ensure all packages in the environment are installed.
│    - Or, if you have FooBarExt checked out for development and have
│      added Bar as a dependency but haven't updated your primary
│      environment's manifest file, try `Pkg.resolve()`.
│    - Otherwise you may need to report an issue with FooBarExt
│    Stacktrace:
│      [1] macro expansion
│        @ ./loading.jl:2262 [inlined]
│      [2] macro expansion
│        @ ./lock.jl:287 [inlined]
│      [3] __require(into::Module, mod::Symbol)
│        @ Base ./loading.jl:2234
│      [4] #invoke_in_world#2
│        @ ./essentials.jl:1082 [inlined]
│      [5] invoke_in_world
│        @ ./essentials.jl:1079 [inlined]
│      [6] require(into::Module, mod::Symbol)
│        @ Base ./loading.jl:2227
│      [7] include
│        @ ./Base.jl:582 [inlined]
│      [8] include_package_for_output(pkg::Base.PkgId, input::String, depot_path::Vector{String}, dl_load_path::Vector{String}, load_path::Vector{String}, concrete_deps::Vector{Pair{Base.PkgId, UInt128}}, source::Nothing)
│        @ Base ./loading.jl:2825
│      [9] top-level scope
│        @ stdin:5
│     [10] eval
│        @ ./boot.jl:439 [inlined]
│     [11] include_string(mapexpr::typeof(identity), mod::Module, code::String, filename::String)
│        @ Base ./loading.jl:2678
│     [12] include_string
│        @ ./loading.jl:2688 [inlined]
│     [13] exec_options(opts::Base.JLOptions)
│        @ Base ./client.jl:327
│     [14] _start()
│        @ Base ./client.jl:558
│    in expression starting at /home/topolarity/repos/julia/Foo/ext/FooBarExt.jl:1
│    in expression starting at stdin:
└ @ Base loading.jl:1543

julia +nightly --project=testenv -q
julia> using Foo, Bar
julia>