refactor(core): remove LocalClient

[m-alisafaee]

Fixes #3083

Test Refactoring:

• Uses a project fixture (an instance of RenkuProject class) to represent a project consistently. Removes the repository fixture.
• Replaces all client, client_*, ... fixtures with project ones
• Replaces load_dataset_with_injection and get_datasets_provenance_with_injection fixtures with functions
• Renames with_injections_manager to with_injection
• Moves migrate.py from renku.core to renku.core.migration
• Removes usages of project_context in the test as much as possible
• Some renaming, spelling fixes, ...

[Panaetius]

There's a few places left that I saw:

• Docstring of transaction_id in project_context.py
• renku.domain_model.datastructures.py, I think only DirectoryTree in this files is actually used and the rest can be deleted.
• tests/utils.py with_dataset, but that can be removed I think.
• And of course the 5 billion client fixtures you already mentioned. There's also svc_client* but I guess in that case, it's actually a client and fine.

[Panaetius]

Good idea on adding path to the project fixture. And thank you for all the little cleanups you did!