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_passes/src/check_attr.rs

@@ -211,7 +211,6 @@ impl CheckAttrVisitor<'_> {
                 sym::deprecated => self.check_deprecated(hir_id, attr, span, target),
                 sym::macro_use | sym::macro_escape => self.check_macro_use(hir_id, attr, target),
                 sym::path => self.check_generic_attr(hir_id, attr, target, Target::Mod),
-                sym::plugin_registrar => self.check_plugin_registrar(hir_id, attr, target),
                 sym::macro_export => self.check_macro_export(hir_id, attr, target),
                 sym::ignore | sym::should_panic => {
                     self.check_generic_attr(hir_id, attr, target, Target::Fn)
@@ -2237,17 +2236,6 @@ impl CheckAttrVisitor<'_> {
         }
     }
 
-    fn check_plugin_registrar(&self, hir_id: HirId, attr: &Attribute, target: Target) {
-        if target != Target::Fn {
-            self.tcx.emit_spanned_lint(
-                UNUSED_ATTRIBUTES,
-                hir_id,
-                attr.span,
-                errors::PluginRegistrar,
-            );
-        }
-    }
-
     fn check_unused_attribute(&self, hir_id: HirId, attr: &Attribute) {
         // Warn on useless empty attributes.
         let note = if matches!(

compiler/rustc_passes/src/errors.rs

@@ -717,10 +717,6 @@ pub enum MacroExport {
     TooManyItems,
 }
 
-#[derive(LintDiagnostic)]
-#[diag(passes_plugin_registrar)]
-pub struct PluginRegistrar;
-
 #[derive(Subdiagnostic)]
 pub enum UnusedNote {
     #[note(passes_unused_empty_lints_note)]