Migrate to mockery's packages configuration

This drops the go:generate lines and adds mocks for all files. This makes generating mocks a lot faster and avoids the need to annotate interfaces.

Previously make generated_files took 75 seconds on my machine; now it only takes 4 seconds despite the increased amount of generated code.

See https://vektra.github.io/mockery/latest/migrating_to_packages/ for more details.