Reimplement the distfile-check as python test inside this git
repository, so we don't need to download a tarball of ci-common.git to
run it. This would not have been nice for running the test with
'pmbootstrap ci', as we don't want it to get downloaded every time the
test runs.

This new implementation is done in less than 40 lines of code, very fast
and doesn't need to create a lot of files in /tmp to build a
"distfiletree".