When Software Heritage persistent identifiers were introduced, they were not
yet abbreviated as SWHIDs.

Now that abbreviation is growing adoption, rename some functions and types in
swh.model.identifiers for consistency:

  - PersistentId -> SWHID

  - persistent_identifier -> swhid

  - parse_persistent_identifier -> parse_swhid

Backward compatibility with previous naming is maintained but deprecation
warnings are introduced to encourage the use of the new names.

Numerous variables in swh.model codebase have also been renamed accordingly.

Also rework and improve documentation.

and user required=True to check that, as it is the preferred way

- blackify all the python files,
- enable black in pre-commit,
- add a black tox environment.

This allows for example to type:

  curl -s https://archive.softwareheritage.org/browse/content/sha1_git:64582b78792cd6c2d67d35da5a11bb80886a6409/raw/ | swh identify
  swh:1:cnt:64582b78792cd6c2d67d35da5a11bb80886a6409	-

related to T1261

Summary: Related to T1261

Test Plan: before and after: 138 tests passed (2 skipped)

Reviewers: #reviewers!

Differential Revision: https://forge.softwareheritage.org/D520

... and add an option to explicitly not follow them, if needed.

Closes T1135

with explicit option to hide it

Closes T1133

Currently only content and directory object types are supported, but more can
be added in the future.

Closes T1039