Move manifest computation functions from identifiers.py to git_objects.py
Since they are used by the vault for non-identifier-related stuff, I think it makes sense to move them to a new module.
identifiers.py is now an empty shell (as !301 (closed) moved SWHID classes out of the module), as all its features were moved to other modules and it only contains reexports and backward-compat functions. Therefore, it should be considered deprecated from now on.
Depends on !302 (closed).
Migrated from D6326 (view on Phabricator)