Make function swh.model.identifiers.parse_swhid more strict

Making it strict by respecting the documented grammar [1]

Currently, it misses some checks on:

• !255 (closed): the context qualifier (origin, visit, ...) so anybody can provide noisy swhid and be fine with it
• !256 (closed): whitespaces that should be refused anywhere in the string and it's not

It should also raise when those invalid SWHIDs are provided [2]

The end goal being to avoid another parsing layer in other swh users of that function.

• [1] https://docs.softwareheritage.org/devel/swh-model/persistent-identifiers.html#syntax

• [2] swh-deposit!134 (closed)

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Migrated from T2769 (view on Phabricator)