cleanup pyproject loading and allow cli relative roots to be specified

[RonnyPfannschmidt]

this also tries to address concerns by @qubidt in #691 / #702

please take a look if this resolves the issue

[baodrate]

looks good! passes all the cases I can come up with

[RonnyPfannschmidt]

@qubidt can you share those, I'd like to have them part of the testsuite

[baodrate]

I only wrote a few, you can see them in the PR I made against the original #702:

https://github.com/GDYendell/setuptools_scm/pull/1/files#diff-6f46d110c3d6f83624aca4044339075d63fdb10acd2b1f0c2ade98204c0a34ec

Of which one (test_repo_cmdline_root_relative_cwd I think) fails against the current master but passes with this PR. If you want to add them to the feel free to copy them. Don't go out of your way to merge my commit or anything, lol

most of the other cases I was referring to were just manual testing I did w/ variations of ./../relative/linked paths for -r and -c. Nothing particularly interesting or useful

[RonnyPfannschmidt]

thanks for the update, time for merge and release then