Move mix dependencies to _build
Rename fetched to loaded and add a manifest_path
Sinplify mechanism for checking and handling locks

In particular, there is no need to pass :lockfile around deps.
We also remove the umbrella handling from the lock.
Ensure archives work with new _build setup
Move rebar and other deps to _build
Share _build path when running commands from umbrella
Ensure rebar dependencies are stored in the proper directory