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-python/src/prefix.rs

@@ -0,0 +1,43 @@
+use std::path::{Path, PathBuf};
+
+use pypi_types::Scheme;
+
+/// A `--prefix` directory into which packages can be installed, separate from a virtual environment
+/// or system Python interpreter.
+#[derive(Debug, Clone)]
+pub struct Prefix(PathBuf);
+
+impl Prefix {
+    /// Return the [`Scheme`] for the `--prefix` directory.
+    pub fn scheme(&self, virtualenv: &Scheme) -> Scheme {
+        Scheme {
+            purelib: self.0.join(&virtualenv.purelib),
+            platlib: self.0.join(&virtualenv.platlib),
+            scripts: self.0.join(&virtualenv.scripts),
+            data: self.0.join(&virtualenv.data),
+            include: self.0.join(&virtualenv.include),
+        }
+    }
+
+    /// Return an iterator over the `site-packages` directories inside the environment.
+    pub fn site_packages(&self, virtualenv: &Scheme) -> impl Iterator<Item = PathBuf> {
+        std::iter::once(self.0.join(&virtualenv.purelib))
+    }
+
+    /// Initialize the `--prefix` directory.
+    pub fn init(&self) -> std::io::Result<()> {
+        fs_err::create_dir_all(&self.0)?;
+        Ok(())
+    }
+
+    /// Return the path to the `--prefix` directory.
+    pub fn root(&self) -> &Path {
+        &self.0
+    }
+}
+
+impl From<PathBuf> for Prefix {
+    fn from(path: PathBuf) -> Self {
+        Self(path)
+    }
+}