Move all code used in gRPC API under pkg/api namespace

This is purely a file move exercise to keep all the gRPC api related code under the same namespace:

• pkg/jobs -> pkg/api/internal/jobs
• pkg/variables -> pkg/api/internal/variables
• pkg/streamer -> pkg/api/internal/streamer
• pkg/internal/test -> pkg/api/internal/test

This is once exception since this could conceivably be used outside of the api

• pkg/syncmap -> pkg/internal/syncmap