use repo `debmagic-pkg` for `debmagic build` in container

[TheJJ]

for development on debmagic-pkg one should be able to launch build containers that use the in-repo debmagic python/rust code instead of a package's build-dependency to debmagic-pkg.
for the python part this could be done by mounting ${debmagic_repo}/packages/debmagic-pkg/src.
and creating /usr/lib/python3/dist-packages/debmagic_pkg.pth pointing to that path.