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)

.github/workflows/ci.yml

@@ -135,7 +135,7 @@ jobs:
 
       - name: "Install required Python versions"
         run: |
-          cargo run toolchain install
+          cargo run python install
 
       - name: "Install cargo nextest"
         uses: taiki-e/install-action@v2
@@ -172,7 +172,7 @@ jobs:
 
       - name: "Install required Python versions"
         run: |
-          cargo run toolchain install
+          cargo run python install
 
       - name: "Install cargo nextest"
         uses: taiki-e/install-action@v2