frontend: add support for optional namespaces in DistGit instances
Fix #1633
With this change, I can use DistGit method with the following
arguments:
Package name: copr-cli
DistGit instance: fedora
and the `clone_url` gets properly generated as
https://src.fedoraproject.org/rpms/copr-cli.git
as it used to.
Additionally, I can use
Package name: python-rpm-macros
Committish: 3.10
DistGit instance: fedora
Namespace: forks/churchyard
and the `clone_url` gets properly evaluated to
https://src.fedoraproject.org/forks/churchyard/rpms/python-rpm-macros.git
While doesn't breaking the validation of DistGit instances for which
the namespace isn't optional. Specifying only
Package name: copr-cli
DistGit instance: fedora-copr
Still returns an error - Can not validate DistGit input: "DistGit
'fedora-copr' requires 'namespace' specified"