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-state/src/lib.rs

@@ -95,8 +95,8 @@ impl StateStore {
 /// are subdirectories of the state store root.
 #[derive(Debug, Clone, Copy, Eq, PartialEq, Hash)]
 pub enum StateBucket {
-    // Managed toolchains
-    Toolchains,
+    // Managed Python installations
+    ManagedPython,
     // Installed tools
     Tools,
 }
@@ -104,7 +104,7 @@ pub enum StateBucket {
 impl StateBucket {
     fn to_str(self) -> &'static str {
         match self {
-            Self::Toolchains => "toolchains",
+            Self::ManagedPython => "python",
             Self::Tools => "tools",
         }
     }