uv-resolver: propagate markers to sibling dependencies in forks
When a fork occurs, we divide not just the dependencies that
provoked a fork into distinct groups, but we also add the
corresponding sibling dependencies to each fork. Previously,
while we track markers on the fork itself, the individual
dependencies that had markers only corresponded to markers
written from the dependency specification.
This meant that the sibling dependencies that got added to
each fork would not themselves have markers attached to them.
This in turn meant they would not have markers associated with
them in the lock file.
In many cases, this is actually okay, because the resolver will
pick a version that is "universal" across all forks in most
cases. But in some cases, this just simply isn't possible as
the marker expressions in the fork can and do influence resolution.
In which case, it is possible for the same package with different
versions to show up in the lock file unconditionally. Which is a
big no-no.
So in this commit, after we determine the forks, we intersect the
markers on each fork with each of its dependencies.
This does seem to balloon the marker expressions in some cases.
I plucked one low hanging fruit to avoid doing `x and x` in
trivial cases. (And this eliminated a portion of the snapshot
diffs.) But some pretty gnarly diffs remain.
This commit also fixes another bug: previously, when we created a fork
to capture the "remaining" universe of an incomplete set of markers, we
left out dependencies that should be included in that fork. We rectify
that here.
Fixes #5086
Partially addresses #4732
