Update the interface for declaring Python download preferences (#5936)

The loose consensus is that "fetch" doesn't have much meaning and that a
boolean flag makes more sense from the command line.

1. Adds `--allow-python-downloads` (hidden, default) and
`--no-python-downloads` to the CLI to quickly enable or disable
downloads
2. Deprecates `--python-fetch` in favor of the options from (1)
3. Removes  `python-fetch` in favor of a `python-downloads` setting
5. Adds a `never` variant to the enum, allowing even explicit installs
to be disabled via the configuration file

## Test plan

I tested this with various `pyproject.toml`-level settings and `uv venv
--preview --python 3.12.2` and `uv python install 3.12.2` with and
without the new CLI flags.

crates/uv-python/src/lib.rs

@@ -2,7 +2,7 @@
 use thiserror::Error;
 
 pub use crate::discovery::{
-    find_python_installations, EnvironmentPreference, Error as DiscoveryError, PythonFetch,
+    find_python_installations, EnvironmentPreference, Error as DiscoveryError, PythonDownloads,
     PythonNotFound, PythonPreference, PythonRequest, PythonSource, VersionRequest,
 };
 pub use crate::environment::PythonEnvironment;