uv-resolver: fix basic case of overlapping markers

[BurntSushi]

Consider the following packse scenario:

[root]
requires = [
  "a>=1.0.0 ; python_version < '3.10'",
  "a>=1.1.0 ; python_version >= '3.10'",
  "a>=1.2.0 ; python_version >= '3.11'",
]

[packages.a.versions."1.0.0"]
[packages.a.versions."1.1.0"]
[packages.a.versions."1.2.0"]

On current main, this produces a dependency on a that looks like
this:

dependencies = [
    { name = "fork-overlapping-markers-basic-a", marker = "python_version < '3.10' or python_version >= '3.11'" },
]

But the marker expression is clearly wrong here, since it implies that
a isn't installed at all for Python 3.10. With this PR, the above
dependency becomes:

dependencies = [
    { name = "fork-overlapping-markers-basic-a" },
]

That is, it's unconditional. Which is I believe correct here since there
aren't any other constraints on which version to select.

The specific bug here is that when we found overlapping dependency
specifications for the same package within a pre-existing fork, we
intersected all of their marker expressions instead of unioning them.
That in turn resulted in incorrect marker expressions.

While this doesn't fix any known bug on the issue tracker (like #4640),
it does appear to fix a couple of our snapshot tests. And fixes a basic
test case I came up with while working on #4732.

For the packse scenario test: astral-sh/packse#206