uv-resolver: implement basic resolver forking
There are still some TODOs/FIXMEs here, but this makes represents a
chunk of the resolver refactoring to enable forking. We don't do any
merging of resolutions yet, so crucially, this code is broken when no
marker environment is provided. But when a marker environment is
provided, this should behave the same as a non-forking resolver. In
particular, `get_dependencies_forking` is just `get_dependencies`
whenever there's a marker environment.
