Factored package manager into separate submodule.

This is the first step toward implementing a command line tool to manage Scaled
packages. That's the first step toward implementing checkout and build
functionality for packages.
Rearrange to eliminate pacman depend on api.

The package manager need not know of Scaled API things. Indeed, we may use it
to run Scaled itself if we're feeling crazy.
More work on new extracted package manager.

After removing a bunch of hackery, I think I want to split everything into
separate top-level projects so that I can have proper dependencies all the way
down.