git.py: Use the configured list of submodules when fetching with aliases

When the source's URL uses an alias (i.e. Source.translate_url()
actually changes the URL), we use the list of submodules defined in the
config as the list of submodules used.
This is because the source would otherwise have to fetch the main repo
to know which submodules it actually has.
This is inconvenient when using mirroring, because when the upstream
repo is unreachable it'll fail when it doesn't have to.

GitMirrors can now be instantiated without having their path and ref
defined, provided the 'parent' mirror is defined. It will retrieve the
path and ref as-needed instead.

If the source's URL does not use an alias, it will behave as normally.