Draft: Improve ability to bootstrap new architectures

A possible solution to #1078929.

Most packages can continue to build-depend on libglib2.0-dev, which is now a metapackage.

Low-level packages that participate in the cross phase of architecture bootstrapping should depend directly on the smaller packages that have been split out from libglib2.0-dev:

• libgio-2.0-dev: ability to link to GIO and its dependencies (GLib, GObject, GThread, GModule)
  • the name was chosen so that if it becomes necessary we could split it further, to break out the "smaller" libraries GLib/GThread/GModule and GObject, without either breaking compatibility or making its name really confusing
  • depends on libglib2.0-0t64, pkgconf and various -dev packages
  • Recommends libgio-2.0-dev-bin but does not currently hard-depend on it
• libgio-2.0-dev-bin: development-oriented CLI tools not related to GObject-Introspection
  • depends on python3-packaging, python3:any and shared libraries
• libglib2.0-bin (pre-existing package): non-development-specific CLI tools
  • depends on libglib2.0-data and shared libraries
• girepository-tools: the ${DEB_HOST_MULTIARCH}-gi-* tools related to GObject-Introspection, which need to be host-architecture tools, run via an exe wrapper such as qemu-user if necessary
  • depends on python3 | qemu-user | qemu-user-static (to be replaced by cross-exe-wrapper when available) and shared libraries

@helmutg, does this do what you need?