uv-resolver: filter dependencies that can't exist in a fork
This commit adds marker expressions to our `Fork` type, which are in
turn passed down into `PubGrubDependencies::from_requirements` to filter
our any dependencies with markers that are disjoint from the fork's
marker expression.
This is necessary to avoid visiting packages in the dependency graph
that can never actually be installed. This is because when a fork is
created in the resolver, it always happens when there are two sibling
dependency specifications on a package with the same name, but with
non-overlapping marker expressions. Each fork corresponds to each
such conflicting dependency specification, and each fork assumes the
the corresponding marker expression as a pre-condition for any future
dependencies considered by it. That is, since the fork represents an
installation path that can only be taken when the corresponding
dependency specification (and its marker expression) is actually used,
it also therefore follows that the marker expression is true. Therefore,
any dependency visited in that fork with a marker expression that cannot
possibly be true when the markers of the fork are true can and ought to
be completely ignored.
