uv-pep508: add MarkerTree::implies
I think Ibraheem had this routine at some point in the past, but
we ended up dropping it because we didn't have a use for it. Well,
now we do!
It turns out that when we generate "conflict markers," they don't
actually take "world knowledge" into account. In particular, there
is "world knowledge" that a particular set of extras cannot be
enabled simultaneously. This in turn allows us to simplify most
conflict markers. If we didn't do this, it's likely that lock files
would become littered with conflict markers whenever any conflicts
are declared.
This is somewhat (although not completely) analogous to how we
"simplify" markers with respect to `requires-python`. That is,
`requires-python` reflects world knowledge that enables markers
to be written more simply than they otherwise would be without
world knowledge.
