"inline test" libraries must be installable if in a package

Hi !

While adding a second package in a project, I hit this issue when specifying which test is attached to which package.

Adding the (package) field on a (inline_tests) library makes it installable. Test-dependencies become regular dependencies (as reported by opam-dune-lint).
That it's a library feels like an implementation detail to me.
I'd like to specify a package, similarly to (package) on a cram test, without making the library installable (eg. to attach the package to an implicit runtest rule rather than the actual library).

While adding a second package, I wished to be able to set (package) on an entire directory tree: https://github.com/ocaml/dune/issues/3255

Start by tracing how (inline_tests) libraries interpret the (package) field and how package dependencies are reported by opam-dune-lint. Compare this behavior with (package) on cram tests and consider the related directory-tree request in issue #3255. Done means a package can be associated with inline tests without making the test library installable or promoting test dependencies to regular dependencies.