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/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,
         )?