Pubgrub stores incompatibilities as (package name, version range) tuples, meaning it needs to clone the package name for each incompatibility, and each non-borrowed operation on incompatibilities. #3673 made me realize that `PubGrubPackage` has gotten large (expensive to copy), so like `Version` and other structs, i've added an `Arc` wrapper around it.

It's a pity clippy forbids `.deref()`, it's less opaque than `&**` and has IDE support (clicking on `.deref()` jumps to the right impl).

## Benchmarks

It looks like this matters most for complex resolutions which, i assume because they carry larger `PubGrubPackageInner::Package` and `PubGrubPackageInner::Extra` types.

```bash
hyperfine --warmup 5 "./uv-main pip compile -q ./scripts/requirements/jupyter.in" "./uv-branch pip compile -q ./scripts/requirements/jupyter.in"
hyperfine --warmup 5 "./uv-main pip compile -q ./scripts/requirements/airflow.in" "./uv-branch pip compile -q ./scripts/requirements/airflow.in"
hyperfine --warmup 5 "./uv-main pip compile -q ./scripts/requirements/boto3.in" "./uv-branch pip compile -q ./scripts/requirements/boto3.in"
```

```
Benchmark 1: ./uv-main pip compile -q ./scripts/requirements/jupyter.in
  Time (mean ± σ):      18.2 ms ±   1.6 ms    [User: 14.4 ms, System: 26.0 ms]
  Range (min … max):    15.8 ms …  22.5 ms    181 runs

Benchmark 2: ./uv-branch pip compile -q ./scripts/requirements/jupyter.in
  Time (mean ± σ):      17.8 ms ±   1.4 ms    [User: 14.4 ms, System: 25.3 ms]
  Range (min … max):    15.4 ms …  23.1 ms    159 runs

Summary
  ./uv-branch pip compile -q ./scripts/requirements/jupyter.in ran
    1.02 ± 0.12 times faster than ./uv-main pip compile -q ./scripts/requirements/jupyter.in
```

```
Benchmark 1: ./uv-main pip compile -q ./scripts/requirements/airflow.in
  Time (mean ± σ):     153.7 ms ±   3.5 ms    [User: 165.2 ms, System: 157.6 ms]
  Range (min … max):   150.4 ms … 163.0 ms    19 runs

Benchmark 2: ./uv-branch pip compile -q ./scripts/requirements/airflow.in
  Time (mean ± σ):     123.9 ms ±   4.6 ms    [User: 152.4 ms, System: 133.8 ms]
  Range (min … max):   118.4 ms … 138.1 ms    24 runs

Summary
  ./uv-branch pip compile -q ./scripts/requirements/airflow.in ran
    1.24 ± 0.05 times faster than ./uv-main pip compile -q ./scripts/requirements/airflow.in
```

```
Benchmark 1: ./uv-main pip compile -q ./scripts/requirements/boto3.in
  Time (mean ± σ):     327.0 ms ±   3.8 ms    [User: 344.5 ms, System: 71.6 ms]
  Range (min … max):   322.7 ms … 334.6 ms    10 runs

Benchmark 2: ./uv-branch pip compile -q ./scripts/requirements/boto3.in
  Time (mean ± σ):     311.2 ms ±   3.1 ms    [User: 339.3 ms, System: 63.1 ms]
  Range (min … max):   307.8 ms … 317.0 ms    10 runs

Summary
  ./uv-branch pip compile -q ./scripts/requirements/boto3.in ran
    1.05 ± 0.02 times faster than ./uv-main pip compile -q ./scripts/requirements/boto3.in
```