It's nice to be able to build from source control HEAD, and
especially for packages under than Chocolate Doom itself. As a start,
define "latest" variants for all the SDL2 packages.
Previously a particular package could only ever be installed to target
or native. Instead, allow any package to be installed to either, with
target being the default. Native packages can only ever have native
dependencies, but target packages can depend on native ones in order to
install tools which they need for their build process.
The only place this is needed is when determining the directory name
that the tar file will be extracted to. This can be done by examining
the .tar file name without needing to know the version itself.
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.
