Fix commit id retrieval

[jtec]

• Addresses https://github.com/jtec/prx-import/issues/2
• Adds a _dirty suffix to the HEAD commit SHA if the working tree differs from HEAD

[plutonheaven]

@jtec I took the initiative to propose some additional changes to your PR.

Setting search_parent_directories=False in the call of git.Repo() was one of the change required.

I also added a function to get the commit ID from the installation of prx as a package, which writes info to a file named direct_url.json somewhere in the .venv folder (PEP 610).

I added a test in https://github.com/jtec/prx-import/pull/3, which seems to work!

[jtec]

@plutonheaven Great! Should we merge this?