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-dispatch/src/lib.rs

@@ -23,10 +23,10 @@ use uv_configuration::{Concurrency, PreviewMode};
 use uv_distribution::DistributionDatabase;
 use uv_git::GitResolver;
 use uv_installer::{Installer, Plan, Planner, Preparer, SitePackages};
+use uv_python::{Interpreter, PythonEnvironment};
 use uv_resolver::{
     ExcludeNewer, FlatIndex, InMemoryIndex, Manifest, OptionsBuilder, PythonRequirement, Resolver,
 };
-use uv_toolchain::{Interpreter, PythonEnvironment};
 use uv_types::{BuildContext, BuildIsolation, EmptyInstalledPackages, HashStrategy, InFlight};
 
 /// The main implementation of [`BuildContext`], used by the CLI, see [`BuildContext`]