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/discovery.rs

@@ -79,12 +79,25 @@ pub enum PythonPreference {
 #[serde(deny_unknown_fields, rename_all = "kebab-case")]
 #[cfg_attr(feature = "clap", derive(clap::ValueEnum))]
 #[cfg_attr(feature = "schemars", derive(schemars::JsonSchema))]
-pub enum PythonFetch {
-    /// Automatically fetch managed Python installations when needed.
+pub enum PythonDownloads {
+    /// Automatically download managed Python installations when needed.
     #[default]
+    #[serde(alias = "auto")]
     Automatic,
-    /// Do not automatically fetch managed Python installations; require explicit installation.
+    /// Do not automatically download managed Python installations; require explicit installation.
     Manual,
+    /// Do not ever allow Python downloads.
+    Never,
+}
+
+impl From<bool> for PythonDownloads {
+    fn from(value: bool) -> Self {
+        if value {
+            PythonDownloads::Automatic
+        } else {
+            PythonDownloads::Never
+        }
+    }
 }
 
 #[derive(Debug, Clone, Copy, PartialEq, Eq, Default)]
@@ -1298,7 +1311,7 @@ impl PythonPreference {
     }
 }
 
-impl PythonFetch {
+impl PythonDownloads {
     pub fn is_automatic(self) -> bool {
         matches!(self, Self::Automatic)
     }