Avoid reparsing wheel URLs (#4947)
## Summary
We currently store wheel URLs in an unparsed state because we don't have
a stable parsed representation to use with rykv. Unfortunately this
means we end up reparsing unnecessarily in a lot of places, especially
when constructing a `Lock`. This PR adds a `UrlString` type that lets us
avoid reparsing without losing the validity of the `Url`.
## Test Plan
Shaves off another ~10 ms from
https://github.com/astral-sh/uv/issues/4860.
```
➜ transformers hyperfine "../../uv/target/profiling/uv lock" "../../uv/target/profiling/baseline lock" --warmup 3
Benchmark 1: ../../uv/target/profiling/uv lock
Time (mean ± σ): 120.9 ms ± 2.5 ms [User: 126.0 ms, System: 80.6 ms]
Range (min … max): 116.8 ms … 125.7 ms 23 runs
Benchmark 2: ../../uv/target/profiling/baseline lock
Time (mean ± σ): 129.9 ms ± 4.2 ms [User: 127.1 ms, System: 86.1 ms]
Range (min … max): 123.4 ms … 141.2 ms 23 runs
Summary
../../uv/target/profiling/uv lock ran
1.07 ± 0.04 times faster than ../../uv/target/profiling/baseline lock
```
