python, cli, frontend: module dist-git instance choice
Before, any module build from provided modulemd.yaml cloned packages
from Fedora's dist-git (if otherwise unspecified in modulemd.yaml).
Newly, there's a commandline option/api option to pick a concrete
instance of dist-git. New dist-git instances can be added dynamically
by manual edit of 'dist_git_instance' table.
The reason for this patch is that the modulemd files that are stored in
dist-git usually don't self-reference the dist-git instance they are
stored in. This caused issues in Red Hat internal copr instance where
we don't self-reference internal dist-git. Now, when properly
configured, we can do `copr build-module ... --distgit redhat`.
Relates: rhbz#1771699