Allow to use previous version of binary mod as dependency

[hackwaly]

Moonyacc need this to do bootstrap process.

[Young-Flash]

• split .mooncakes into .mooncakes/source & .mooncakes/binary
• allow previous version of binary mod, which in .mooncakes/binary

cc @lynzrand

[lynzrand]

NB: The whole binary dependency scheme might need an overhaul.

Specifically:

• Version numbers in .mooncakes/* package paths, to support multiple versions of the same package at once.
• Overhaul of the MVS to add binary deps that don't coerce to local modules (need review of the code)
• Reference binary deps in build graph directly, instead of building them on installation (which spins up another moon instance and perform quirky path fiddling that nobody knows who's depending on)