Make relative baseline file paths relative to the current pyproject.toml #14

Currently flakehell searches for baseline files in the current working directory. It doesn't play nice together with pyproject.toml search logic.

Consider the following example:

project/
    pyproject.toml # baseline = flakehell.baseline
    flakehell.baseline
    subdir1/
        ...
    subdir2/
        ...

If flakehell executed in subdir1 or subdir2 it finds project-level pyproject.toml but fails to find flakehell.baseline as it tries to find it in the current directory.

The proposal is to make it search files referenced in pyproject.toml relative to pyproject.toml location. If referenced files have relative paths, of course.

The same should be done for the base files, I guess (didn't check the behavior with them).

If you think this is okay, I can prepare a PR with these changes.