source.py: Added `primary` argument to URL marking APIs

The Source must now mention whether the marked or translated
URL is "primary" or not. Even when a Source may have multiple
URLs, the auxilliary URLs are derived from the primary one, not
only is this true for git, but it is mandated by our tracking
API which assumes there is a primary URL.

This adjusts the `git` source and the test `fetch_source.py` source
to behave properly and advertize it's primary URL properly.

This is a part of #620