Remove unnecessary uses of DashMap and Arc (#3413)

## Summary

All of the resolver code is run on the main thread, so a lot of the
`Send` bounds and uses of `DashMap` and `Arc` are unnecessary. We could
also switch to using single-threaded versions of `Mutex` and `Notify` in
some places, but there isn't really a crate that provides those I would
be comfortable with using.

The `Arc` in `OnceMap` can't easily be removed because of the uv-auth
code which uses the
[reqwest-middleware](https://docs.rs/reqwest-middleware/latest/reqwest_middleware/trait.Middleware.html)
crate, that seems to adds unnecessary `Send` bounds because of
`async-trait`. We could duplicate the code and create a `OnceMapLocal`
variant, but I don't feel that's worth it.

crates/uv-dispatch/src/lib.rs

@@ -314,7 +314,7 @@ impl<'a> BuildContext for BuildDispatch<'a> {
             build_kind,
             self.build_extra_env_vars.clone(),
         )
-        .boxed()
+        .boxed_local()
         .await?;
         Ok(builder)
     }