Make coprdirs have their repo available in the buildroot

[FrostyX]

This is useful in general but especially for pull requests that build multiple packages which depend on each other (e.g. python-copr and copr-cli). Up until now, it had to be workarounded by building the dependency into the main copr repository.

[praiskup]

This would bring security issue into the src.fedoraproject.org integration (the pagure events handler).... a single Copr project may integrate multiple packages from s.f.o. being developed by multiple packagers from s.f.o, and all such integrated packages can build from the corresponding "pull-request 1" (separate PRs, heading the same CoprDir!) and then, the package builds are done against each other.

We need to solve the s.f.o integration first, somehow.