Change "toolchain" to "python" (#4735)

Whew this is a lot.

The user-facing changes are:

- `uv toolchain` to `uv python` e.g. `uv python find`, `uv python
install`, ...
- `UV_TOOLCHAIN_DIR` to` UV_PYTHON_INSTALL_DIR`
- `<UV_STATE_DIR>/toolchains` to `<UV_STATE_DIR>/python` (with
[automatic
migration](https://github.com/astral-sh/uv/pull/4735/files#r1663029330))
- User-facing messages no longer refer to toolchains, instead using
"Python", "Python versions" or "Python installations"

The internal changes are:

- `uv-toolchain` crate to `uv-python`
- `Toolchain` no longer referenced in type names
- Dropped unused `SystemPython` type (previously replaced)
- Clarified the type names for "managed Python installations"
- (more little things)

crates/uv-dev/src/build.rs

@@ -16,8 +16,8 @@ use uv_configuration::{
 };
 use uv_dispatch::BuildDispatch;
 use uv_git::GitResolver;
+use uv_python::{EnvironmentPreference, PythonEnvironment, PythonRequest};
 use uv_resolver::{FlatIndex, InMemoryIndex};
-use uv_toolchain::{EnvironmentPreference, PythonEnvironment, ToolchainRequest};
 use uv_types::{BuildContext, BuildIsolation, InFlight};
 
 #[derive(Parser)]
@@ -68,8 +68,8 @@ pub(crate) async fn build(args: BuildArgs) -> Result<PathBuf> {
     let index_urls = IndexLocations::default();
     let index_strategy = IndexStrategy::default();
     let setup_py = SetupPyStrategy::default();
-    let toolchain = PythonEnvironment::find(
-        &ToolchainRequest::default(),
+    let python = PythonEnvironment::find(
+        &PythonRequest::default(),
         EnvironmentPreference::OnlyVirtual,
         &cache,
     )?;
@@ -78,7 +78,7 @@ pub(crate) async fn build(args: BuildArgs) -> Result<PathBuf> {
     let build_dispatch = BuildDispatch::new(
         &client,
         &cache,
-        toolchain.interpreter(),
+        python.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::{EnvironmentPreference, PythonEnvironment, ToolchainRequest};
+use uv_python::{EnvironmentPreference, PythonEnvironment, PythonRequest};
 
 #[derive(Parser)]
 pub(crate) struct CompileArgs {
@@ -21,7 +21,7 @@ pub(crate) async fn compile(args: CompileArgs) -> anyhow::Result<()> {
         python
     } else {
         let interpreter = PythonEnvironment::find(
-            &ToolchainRequest::default(),
+            &PythonRequest::default(),
             EnvironmentPreference::OnlyVirtual,
             &cache,
         )?