Remove support for compiler plugins.

They've been deprecated for four years.

This commit includes the following changes.
- It eliminates the `rustc_plugin_impl` crate.
- It changes the language used for lints in
  `compiler/rustc_driver_impl/src/lib.rs` and
  `compiler/rustc_lint/src/context.rs`. External lints are now called
  "loaded" lints, rather than "plugins" to avoid confusion with the old
  plugins. This only has a tiny effect on the output of `-W help`.
- E0457 and E0498 are no longer used.
- E0463 is narrowed, now only relating to unfound crates, not plugins.
- The `plugin` feature was moved from "active" to "removed".
- It removes the entire plugins chapter from the unstable book.
- It removes quite a few tests, mostly all of those in
  `tests/ui-fulldeps/plugin/`.

Closes #29597.

compiler/rustc_feature/src/builtin_attrs.rs

@@ -417,24 +417,6 @@ pub const BUILTIN_ATTRIBUTES: &[BuiltinAttribute] = &[
         naked_functions, experimental!(naked)
     ),
 
-    // Plugins:
-    BuiltinAttribute {
-        name: sym::plugin,
-        only_local: false,
-        type_: CrateLevel,
-        template: template!(List: "name"),
-        duplicates: DuplicatesOk,
-        gate: Gated(
-            Stability::Deprecated(
-                "https://github.com/rust-lang/rust/pull/64675",
-                Some("may be removed in a future compiler version"),
-            ),
-            sym::plugin,
-            "compiler plugins are deprecated",
-            cfg_fn!(plugin)
-        ),
-    },
-
     // Testing:
     gated!(
         test_runner, CrateLevel, template!(List: "path"), ErrorFollowing, custom_test_frameworks,