Add git directory loader to ingest git tree at specific references

This provides the method fetch_artifact. This clones a repository at a specific branch, tag or commit and ingests the DAG objects from the resulting directory tree.

Done:

• Bump loader.core for an evolution and a fix [2]
• Reuse loader.core.tests' fetch_nar_extids_from_checksums
• Adds tests on the loader directory ingestion (notably failing cases + other nominal case on various distinct commit, tag, branch)
• Declare the new celery task

Refs. swh/meta#4979 (closed)

Depends on swh-loader-core!469 (merged)

[2] Depends on swh-loader-core!471 (merged)

Co-Authored: Antoine Lambert antoine.lambert@inria.fr