By default we build the stable variant but provide a user-editable
list of packages which will be built at the latest variant instead.
Rename chocolate-doom-git package to chocolate-doom and have it use
the latest variant for the current Git build.
These modules encapsulate individual pieces of functionality and make
the code far more maintainable. This is a pretty big refactoring but
overall a big win.
We need to distinguish between tools we compile to use as part of the
build process, and things we build for the eventual target. Install
these into separate install directories; for now, pkg-config is the
only one of these.