Consolidate concurrency limits (#3493)

## Summary

This PR consolidates the concurrency limits used throughout `uv` and
exposes two limits, `UV_CONCURRENT_DOWNLOADS` and
`UV_CONCURRENT_BUILDS`, as environment variables.

Currently, `uv` has a number of concurrent streams that it buffers using
relatively arbitrary limits for backpressure. However, many of these
limits are conflated. We run a relatively small number of tasks overall
and should start most things as soon as possible. What we really want to
limit are three separate operations:
- File I/O. This is managed by tokio's blocking pool and we should not
really have to worry about it.
- Network I/O.
- Python build processes.

Because the current limits span a broad range of tasks, it's possible
that a limit meant for network I/O is occupied by tasks performing
builds, reading from the file system, or even waiting on a `OnceMap`. We
also don't limit build processes that end up being required to perform a
download. While this may not pose a performance problem because our
limits are relatively high, it does mean that the limits do not do what
we want, making it tricky to expose them to users
(https://github.com/astral-sh/uv/issues/1205,
https://github.com/astral-sh/uv/issues/3311).

After this change, the limits on network I/O and build processes are
centralized and managed by semaphores. All other tasks are unbuffered
(note that these tasks are still bounded, so backpressure should not be
a problem).

crates/uv-configuration/src/concurrency.rs

@@ -0,0 +1,35 @@
+use std::num::NonZeroUsize;
+
+/// Concurrency limit settings.
+#[derive(Copy, Clone, Debug)]
+pub struct Concurrency {
+    /// The maximum number of concurrent downloads.
+    ///
+    /// Note this value must be non-zero.
+    pub downloads: usize,
+    /// The maximum number of concurrent builds.
+    ///
+    /// Note this value must be non-zero.
+    pub builds: usize,
+}
+
+impl Default for Concurrency {
+    fn default() -> Self {
+        Concurrency {
+            downloads: Concurrency::DEFAULT_DOWNLOADS,
+            builds: Concurrency::default_builds(),
+        }
+    }
+}
+
+impl Concurrency {
+    // The default concurrent downloads limit.
+    pub const DEFAULT_DOWNLOADS: usize = 50;
+
+    // The default concurrent builds limit.
+    pub fn default_builds() -> usize {
+        std::thread::available_parallelism()
+            .map(NonZeroUsize::get)
+            .unwrap_or(1)
+    }
+}

crates/uv-configuration/src/lib.rs

@@ -1,5 +1,6 @@
 pub use authentication::*;
 pub use build_options::*;
+pub use concurrency::*;
 pub use config_settings::*;
 pub use constraints::*;
 pub use name_specifiers::*;
@@ -10,6 +11,7 @@ pub use target_triple::*;
 
 mod authentication;
 mod build_options;
+mod concurrency;
 mod config_settings;
 mod constraints;
 mod name_specifiers;