Merge uv-toolchain and uv-interpreter (#3265)

Moves all of `uv-toolchain` into `uv-interpreter`. We may split these
out in the future, but the refactoring I want to do for interpreter
discovery is easier if I don't have to deal with entanglement. Includes
some restructuring of `uv-interpreter`.

Part of #2386

crates/uv-dev/src/fetch_python.rs

@@ -13,7 +13,7 @@ use tokio::time::Instant;
 use tracing::{info, info_span, Instrument};
 
 use uv_fs::Simplified;
-use uv_toolchain::{
+use uv_interpreter::managed::{
     DownloadResult, Error, PythonDownload, PythonDownloadRequest, TOOLCHAIN_DIRECTORY,
 };