Build debugging repl

[andrewchambers]

During a package build it seems important to be able to do a few things:

• Drop to a repl (This should invalidate the build).
• Launch a regular terminal from a debug repl.
• Get local access to the build directory (perhaps via sshfs?).

This should work the same for both local builds and remote builds.

I envision this feature to work similarly to a the fetch.sock but when --debug is set, there is also a debug.sock that forwards connections to a local daemon, (exactly like the fetch server).

[andrewchambers]

Stretch goal would be save/rewind. This might be as simple as rsync + saving environment variables.