Mock can build RPMs directly from the dist-git checkout. This isn't
well documented in Mock, but e.g. Copr already does
this since af731d43f17f6ef299.

Generating SRPM on host is tricky, it implies spec file parsing by RPM,
which may easily fail (e.g. %patchN statements break rpmbuild -bs).

FTR, even if we do fedpkg srpm for Mock, the SRPM is extracted first, and
then re-built. Mock then installs BuildRequires using that "native" src.rpm, and so on.