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/bench/Cargo.toml

@@ -40,7 +40,7 @@ uv-configuration = { workspace = true }
 uv-dispatch = { workspace = true }
 uv-distribution = { workspace = true }
 uv-git = { workspace = true }
-uv-toolchain = { workspace = true }
+uv-python = { workspace = true }
 uv-resolver = { workspace = true }
 uv-types = { workspace = true }
 

crates/bench/benches/uv.rs

@@ -5,8 +5,8 @@ use bench::criterion::{criterion_group, criterion_main, measurement::WallTime, C
 use pypi_types::Requirement;
 use uv_cache::Cache;
 use uv_client::RegistryClientBuilder;
+use uv_python::PythonEnvironment;
 use uv_resolver::Manifest;
-use uv_toolchain::PythonEnvironment;
 
 fn resolve_warm_jupyter(c: &mut Criterion<WallTime>) {
     let runtime = &tokio::runtime::Builder::new_current_thread()
@@ -87,11 +87,11 @@ mod resolver {
     use uv_dispatch::BuildDispatch;
     use uv_distribution::DistributionDatabase;
     use uv_git::GitResolver;
+    use uv_python::PythonEnvironment;
     use uv_resolver::{
         FlatIndex, InMemoryIndex, Manifest, OptionsBuilder, PythonRequirement, ResolutionGraph,
         Resolver,
     };
-    use uv_toolchain::PythonEnvironment;
     use uv_types::{BuildIsolation, EmptyInstalledPackages, HashStrategy, InFlight};
 
     static MARKERS: Lazy<MarkerEnvironment> = Lazy::new(|| {