refactor: Turn RuleSelector into a newtype around RuleCodePrefix

Yet another refactor to let us implement the many-to-many mapping
between codes and rules in a prefix-agnostic way.

We want to break up the RuleCodePrefix[1] enum into smaller enums.
To facilitate that this commit  introduces a new wrapping type around
RuleCodePrefix so that we can start breaking it apart.

[1]: Actually `RuleCodePrefix` is the previous name of the autogenerated
enum ... I renamed it in b19258a243 to
RuleSelector since `ALL` isn't a prefix. This commit now renames it back
but only because the new `RuleSelector` wrapper type, introduced in this
commit, will let us move the `ALL` variant from `RuleCodePrefix` to
`RuleSelector` in the next commit.

ruff_cli/src/cli.rs

@@ -3,11 +3,12 @@ use std::path::PathBuf;
 use clap::{command, Parser};
 use regex::Regex;
 use ruff::logging::LogLevel;
-use ruff::registry::{Rule, RuleSelector};
+use ruff::registry::Rule;
 use ruff::resolver::ConfigProcessor;
 use ruff::settings::types::{
     FilePattern, PatternPrefixPair, PerFileIgnore, PythonVersion, SerializationFormat,
 };
+use ruff::RuleSelector;
 use rustc_hash::FxHashMap;
 
 #[derive(Debug, Parser)]