Rename uv-interpreter crate to uv-toolchain (#4120)

In preparation for managed toolchains #2607, just renames the crate to
something broader.

See #4121 and https://github.com/astral-sh/uv/pull/4138 to see the final
intent.

crates/uv-resolver/src/python_requirement.rs

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