Refactor uv-toolchain types (#4121)

Extends #4120 
Part of #2607 

There should be no behavior changes here. Restructures the discovery API
to be focused on a toolchain first perspective in preparation for
exposing a `find_or_fetch` method for toolchains in
#4138.

crates/bench/benches/uv.rs

@@ -15,7 +15,7 @@ fn resolve_warm_jupyter(c: &mut Criterion<WallTime>) {
         .unwrap();
 
     let cache = &Cache::from_path("../../.cache").init().unwrap();
-    let venv = PythonEnvironment::from_virtualenv(cache).unwrap();
+    let venv = PythonEnvironment::from_root("../../.venv", cache).unwrap();
     let client = &RegistryClientBuilder::new(cache.clone()).build();
     let manifest = &Manifest::simple(vec![Requirement::from(
         pep508_rs::Requirement::from_str("jupyter").unwrap(),
@@ -44,7 +44,7 @@ fn resolve_warm_airflow(c: &mut Criterion<WallTime>) {
         .unwrap();
 
     let cache = &Cache::from_path("../../.cache").init().unwrap();
-    let venv = PythonEnvironment::from_virtualenv(cache).unwrap();
+    let venv = PythonEnvironment::from_root("../../.venv", cache).unwrap();
     let client = &RegistryClientBuilder::new(cache.clone()).build();
     let manifest = &Manifest::simple(vec![
         Requirement::from(pep508_rs::Requirement::from_str("apache-airflow[all]").unwrap()),

crates/uv-dev/src/build.rs

@@ -16,7 +16,7 @@ use uv_configuration::{
 use uv_dispatch::BuildDispatch;
 use uv_git::GitResolver;
 use uv_resolver::{FlatIndex, InMemoryIndex};
-use uv_toolchain::PythonEnvironment;
+use uv_toolchain::Toolchain;
 use uv_types::{BuildContext, BuildIsolation, InFlight};
 
 #[derive(Parser)]
@@ -65,12 +65,12 @@ pub(crate) async fn build(args: BuildArgs) -> Result<PathBuf> {
     let index = InMemoryIndex::default();
     let index_urls = IndexLocations::default();
     let setup_py = SetupPyStrategy::default();
-    let venv = PythonEnvironment::from_virtualenv(&cache)?;
+    let toolchain = Toolchain::find_virtualenv(&cache)?;
 
     let build_dispatch = BuildDispatch::new(
         &client,
         &cache,
-        venv.interpreter(),
+        toolchain.interpreter(),
         &index_urls,
         &flat_index,
         &index,

crates/uv-dev/src/compile.rs

@@ -3,7 +3,7 @@ use std::path::PathBuf;
 use clap::Parser;
 use tracing::info;
 use uv_cache::{Cache, CacheArgs};
-use uv_toolchain::PythonEnvironment;
+use uv_toolchain::Toolchain;
 
 #[derive(Parser)]
 pub(crate) struct CompileArgs {
@@ -20,8 +20,8 @@ pub(crate) async fn compile(args: CompileArgs) -> anyhow::Result<()> {
     let interpreter = if let Some(python) = args.python {
         python
     } else {
-        let venv = PythonEnvironment::from_virtualenv(&cache)?;
-        venv.python_executable().to_path_buf()
+        let interpreter = Toolchain::find_virtualenv(&cache)?.into_interpreter();
+        interpreter.sys_executable().to_path_buf()
     };
 
     let files = uv_installer::compile_tree(