QualifiedSWHID: Fix (de)serialization of 'origin' qualifier
Having the escaped URL in swhid.origin is inconsistent with self.path
(which is always escaped) and never what we want, because it is only
useful while serializing, which is already handled by __str__.
This led to swh-indexer#4738 (closed)
where swh-deposit parsed a qualified SWHID, then used .origin to get
an origin URL.
Additionally, as serialization always escapes the origin qualifier,
this means that deserializing then re-serializing a qualified SWHID
would double-escape it.
Finally, fixing this made the test uncover that % was not escaped
while serializing, while ; was, leading to incorrect (and ambiguous)
escaped URLs.