Split out Composer Stager wiring into Package Manager submodule

Problem/Motivation

There seems to be a consensus developing that there should be three modules as part of this whole AU/PB initiative set: a package manager (the API for managing the code base via Composer); an extension manager that does the updating (i.e., Automatic Updates, the module); and a project browser. To get things into that configuration, let's split the Composer Stager support stuff off into its own submodule, called Package Manager.