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-resolver/Cargo.toml

@@ -27,7 +27,7 @@ uv-client = { workspace = true }
 uv-configuration = { workspace = true }
 uv-distribution = { workspace = true }
 uv-git = { workspace = true }
-uv-toolchain = { workspace = true }
+uv-python = { workspace = true }
 uv-normalize = { workspace = true }
 uv-types = { workspace = true }
 uv-warnings = { workspace = true }

crates/uv-resolver/src/python_requirement.rs

@@ -1,6 +1,6 @@
 use pep440_rs::VersionSpecifiers;
 use pep508_rs::{MarkerTree, StringVersion};
-use uv_toolchain::{Interpreter, PythonVersion};
+use uv_python::{Interpreter, PythonVersion};
 
 use crate::{RequiresPython, RequiresPythonBound};