mocks: Reorganize mock definitions into multiple files

Created by: efritz

Move all _test.go generated files into mockgen.test.yaml and all others into mockgen.temp.yaml. We should try to minimize the number of non-test mocks we generate here, so the second file should go down to zero eventually (wishful thinking).

Other developers are free to add additional team/domain-specific mock files (in other layers of the file hierarchy as well) where meaningful.

Test plan

Ran locally to ensure the same content was generated.