Switch to Current-Thread Tokio Runtime (#4934)
## Summary
Move completely off tokio's multi-threaded runtime. We've slowly been
making changes to be smarter about scheduling in various places instead
of depending on tokio's general purpose work-stealing, notably
https://github.com/astral-sh/uv/pull/3627 and
https://github.com/astral-sh/uv/pull/4004. We now no longer benefit from
the multi-threaded runtime, as we run on all I/O on the main thread.
There's one remaining instance of `block_in_place` that can be swapped
for `rayon::spawn`.
This change is a small performance improvement due to removing some
unnecessary overhead of the multi-threaded runtime (e.g. spawning
threads), but nothing major. It also removes some noise from profiles.
## Test Plan
```
Benchmark 1: ./target/profiling/uv (resolve-warm)
Time (mean ± σ): 14.9 ms ± 0.3 ms [User: 3.0 ms, System: 17.3 ms]
Range (min … max): 14.1 ms … 15.8 ms 169 runs
Benchmark 2: ./target/profiling/baseline (resolve-warm)
Time (mean ± σ): 16.1 ms ± 0.3 ms [User: 3.9 ms, System: 18.7 ms]
Range (min … max): 15.1 ms … 17.3 ms 162 runs
Summary
./target/profiling/uv (resolve-warm) ran
1.08 ± 0.03 times faster than ./target/profiling/baseline (resolve-warm)
```
