Alter fallback when source-root based package path discovery when a source root containing the module directory is not found

[akamat10]

Current problem

A common setup for repos is to use the src layout discussed here. Currently, for src layouts, we could set --source-roots to be src when calling pylint with the source. This avoids the legacy issue of import discovery if there are namespace packages. One issue with the solution is that, if the user wants to also lint say the test directory in the repo, this solution does not work. That is because currently if a source root doesn't contain the module or directory passed to pylint for linting, it will fallback to legacy behavior of walking the directory. This will lead to import errors when linting the test files.

Desired solution

Alter the fallback logic when source-root containing module directory is not found here to return a list of source-roots. This requires changing the function return type to be a Sequence[str] instead of str. For src layout, there would only be one source-root. This would neatly work for the above case. Let me know your thoughts.

Sample project layout (borrowed from https://docs.pytest.org/en/7.1.x/explanation/goodpractices.html)

pyproject.toml
src/
    mypkg/
        __init__.py
        app.py
        view.py
tests/
    test_app.py
    test_view.py
    ...

After the change, pylint command would be:

pylint --source-roots=src .

Note that this would be a general solution for a project with or without implicit namespace packages.

Additional context

No response

[akamat10]

Implementation provided in #9967. Doesn't seem to break any regressions tests or files other than having to make changes since return type changes from str to Sequence[str]