adds "from" path to package location

[gzt5142]

The patch in this pull request adds support for the 'from' key in a package description.

Why?

I typically use 'src' layout for python projects:

project-name/
    README.md
    ...etc...
    pyproject.toml
    src/
        project_name/
            __init__.py
    tests/
        conftest.py
        test_stuff.py

With this line in my pyproject.toml:

packages = [  
    { include = "project_name", from = "src" },
]

The plugin was not finding the __init__.py with this sort of project layout, although poetry was finding the package just fine. The tweaks in this PR will follow the path indicated in the from key in a package description as above.

[KoenBaak]

Would also really like this feature ! 😄