uv-pep508: fix disjointness bug
This commit fixes a bug where disjointness checking didn't always
satisfy commutativity. And it *should*. The `is_disjoint_commutative`
test added here demonstrates a regression test. Before this commit,
its second assertion failed.
That is, given `m1 = extra == "A" and extra != "B"` and
`m2 = extra == "A"`, we were saying that m1 was disjoint with
m2 (wrong) but that m2 was not disjoint with m1 (right).
It turned out that this was a "simple" matter of not using the
correct parent node when calling negation. Likely just a
transcription snafu.
This bug does not seem restricted in scope to extras, which is
how I found it, so it's not clear why we haven't noticed it until
now. I noticed it because I was formulating markers in a similar
format for resolver forking based on conflicting extras, and this
resulted in incorrectly filtering out dependencies due to `is_disjoint`
returning a false positive.
