Consider system packages as installed if the venv includes them

[abompard]

This changeset adds a getter to env.virtual_env.VirtualEnv to check whether it has access to system packages, and overrides the is_path_relative_to_lib method to take it into account.

This will prevent Poetry from reinstalling system packages in the venv when they are already installed with a compatible version.

Resolves: #6035

• Added tests for changed code.
• Updated documentation for changed code. (N/A)

[radoering]

IMO, it would be nice to have a test in test_installed_repository.py that checks that a package from system site packages does not have source_type == "directory".

[abompard]

IMO, it would be nice to have a test in test_installed_repository.py that checks that a package from system site packages does not have source_type == "directory".

I thought that as well, but in that file the virtual environment is entirely mocked, so I thought it wouldn't be terribly useful as I didn't change any of the calls in repository. What do you think?

[radoering]

I think it makes sense if we set up the test a little differently. Please take a look at the test I just added.

[abompard]

I think it makes sense if we set up the test a little differently. Please take a look at the test I just added.

Oh yeah that's a good idea! Thanks :-)

[github-actions]

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