Add a ViolationMetadata::rule method (#18234)

Summary
--

This PR adds a macro-generated method to retrieve the `Rule` associated
with a given `Violation` struct, which makes it substantially cheaper
than parsing from the rule name. The rule is then converted to a
`NoqaCode` for storage on the `Message` (and eventually on the new
diagnostic type). The `ViolationMetadata::rule_name` method was now
unused, so the `rule` method replaces it.

Several types had to be moved from the `ruff_diagnostics` crate to the
`ruff_linter` crate to make this work, namely the `Violation` traits and
the old `Diagnostic` type, which had a constructor generic over a
`Violation`.

It's actually a fairly small PR, minus the hundreds of import changes.
The main changes are in these files:

-
[crates/ruff_linter/src/message/mod.rs](https://github.com/astral-sh/ruff/pull/18234/files#diff-139754ea310d75f28307008d21c771a190038bd106efe3b9267cc2d6c0fa0921)
-
[crates/ruff_diagnostics/src/lib.rs](https://github.com/astral-sh/ruff/pull/18234/files#diff-8e8ea5c586935bf21ea439f24253fcfd5955d2cb130f5377c2fa7bfee3ea3a81)
-
[crates/ruff_linter/src/diagnostic.rs](https://github.com/astral-sh/ruff/pull/18234/files#diff-1d0c9aad90d8f9446079c5be5f284150d97797158715bd9729e6f1f70246297a)
-
[crates/ruff_linter/src/lib.rs](https://github.com/astral-sh/ruff/pull/18234/files#diff-eb93ef7e78a612f5fa9145412c75cf6b1a5cefba1c2233e4a11a880a1ce1fbcc)

Test Plan
--

Existing tests

scripts/add_rule.py

@@ -92,9 +92,9 @@ def main(*, name: str, prefix: str, code: str, linter: str) -> None:
     with (rules_dir / f"{rule_name_snake}.rs").open("w") as fp:
         fp.write(
             f"""\
-use ruff_diagnostics::Violation;
 use ruff_macros::{{ViolationMetadata, derive_message_formats}};
 
+use crate::Violation;
 use crate::checkers::ast::Checker;
 
 /// ## What it does