Related to T2610

move most import statements in functions.

Related to T2575.

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	-

This is a fix to workaround pip's inability to correctly solve
extra requirements (swh-model depends on swh-core[], but if other
packages depend on swh-model and swh-core[http], the 'http' extra
does not always get installed).

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

Closes T1135

Closes T1134

with explicit option to hide it

Closes T1133

Close T1104
Close D346

Related T1104

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

Closes T1039