Add incompatibility from proxy to base package

[konstin]

Add an incompatibility that lets pubgrub skip of marker packages when the base package already has an incompatible version to improve the error messages (#15199).

The change is also a small perf improvement. Overall this should be able to improve performance in slow cases by avoiding trying proxy package versions that are impossible anyway, for a (ideally very small cost) for tracking the additional incompatibility and tracking the base package for each proxy package.

$ hhyperfine --warmup 2 "uv pip compile --universal scripts/requirements/airflow.in" "target/release/uv pip compile --universal scripts/requirements/airflow.in"
Benchmark 1: uv pip compile --universal scripts/requirements/airflow.in
  Time (mean ± σ):     145.5 ms ±   3.9 ms    [User: 154.7 ms, System: 140.7 ms]
  Range (min … max):   139.2 ms … 153.4 ms    20 runs
 
Benchmark 2: target/release/uv pip compile --universal scripts/requirements/airflow.in
  Time (mean ± σ):     128.7 ms ±   5.5 ms    [User: 141.9 ms, System: 137.3 ms]
  Range (min … max):   121.8 ms … 142.0 ms    23 runs
 
Summary
  target/release/uv pip compile --universal scripts/requirements/airflow.in ran
    1.13 ± 0.06 times faster than uv pip compile --universal scripts/requirements/airflow.in

This implementation is the basic version: When we see a proxy foo{...}>=x,<y we add a dependency edge foo{...}>=x,<y -> foo>=x,<y. There are several way to extend this, which likely help more with performance than with error messages.

One idea is that if we see foo{...}>=x,<y but we already made a selection for foo==z outside that range, we can insert a dependency foo{...}!=z -> foo!=z. This avoids trying any version of the proxy package except the version that matches our previous selection.

Another is that if we see a dependency foo>=x,<y, we also add foo{...}>=x,y -> foo>=x,<y. This allows backtracking beyond foo immediately if all version of foo{...}>=x,<y are incompatible, since foo{...}>=x,<y incompatible -> foo>=x,<y incompatible -> the package that depended of foo>=x,<y is incompatible.

The cost for each of these operations is tracking an additional incompatibility per virtual package. An alternative approach is to only add the incompatibility lazily, only when we've tried several version of the virtual package already. This needs to be weighed of with the better error messages that the incompatibility gives, we unfortunately have only few large reference examples.

Requires astral-sh/pubgrub#45

Closes #15199

[konstin]

Tagging both zanie and andrew because in an older version I broke a conflict test by accidentally including dependency groups, we should ensure that there are no other gotchas with dependency groups and/or conflicts.

[zanieb]

Cool. I do not have a deep understanding of the implications here — it makes sense to have another reviewer.

[BurntSushi]

I think this makes sense to me! This is a nice improvement. :-)

[BurntSushi]

Oh nice!

[BurntSushi]

Much better!

[konstin]

No changes in any tested package.