deposit.parser: Add parse_swh_reference function
Not used yet, simply opened and check coverage is good.
This will ease the detection of a swhid reference or an origin within the <swh:deposit /> xml block.
This is a first step towards adding more support to metadata-only deposit.
Related to #2537 (closed)
Test Plan
tox
Migrated from D4442 (view on Phabricator)