Arc-wrap `PubGrubPackage` for cheap cloning in pubgrub (#3688)
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.
https://github.com/astral-sh/uv/pull/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
```
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