We currently run pylint on all files we find in the checkout. This is
particularly annoying when we have osbuild-stores in the checkout, which
then contain lots of python files in their stored trees.
Change the pylint test to use `git ls-tree` to find all files in the
index and then only run pylint against the subset that we are interested
in.
Introduce a third test-group called `src` alongside `mod` and `run.
This will contain tests that run against the source code of osbuild.
This initial commit introduces `test/src/test_pylint.py` which will run
the python linter against all our sources.