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)
```

crates/uv-dev/src/main.rs

@@ -84,7 +84,7 @@ async fn run() -> Result<()> {
     Ok(())
 }
 
-#[tokio::main]
+#[tokio::main(flavor = "current_thread")]
 async fn main() -> ExitCode {
     let (duration_layer, _guard) = if let Ok(location) = env::var("TRACING_DURATIONS_FILE") {
         let location = PathBuf::from(location);