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_lint/src/context.rs

@@ -109,7 +109,7 @@ struct LintAlias {
 
 struct LintGroup {
     lint_ids: Vec<LintId>,
-    from_plugin: bool,
+    is_loaded: bool,
     depr: Option<LintAlias>,
 }
 
@@ -160,9 +160,7 @@ impl LintStore {
                 // Don't display deprecated lint groups.
                 depr.is_none()
             })
-            .map(|(k, LintGroup { lint_ids, from_plugin, .. })| {
-                (*k, lint_ids.clone(), *from_plugin)
-            })
+            .map(|(k, LintGroup { lint_ids, is_loaded, .. })| (*k, lint_ids.clone(), *is_loaded))
     }
 
     pub fn register_early_pass(
@@ -221,7 +219,7 @@ impl LintStore {
                         .entry(edition.lint_name())
                         .or_insert(LintGroup {
                             lint_ids: vec![],
-                            from_plugin: lint.is_plugin,
+                            is_loaded: lint.is_loaded,
                             depr: None,
                         })
                         .lint_ids
@@ -234,7 +232,7 @@ impl LintStore {
                         .entry("future_incompatible")
                         .or_insert(LintGroup {
                             lint_ids: vec![],
-                            from_plugin: lint.is_plugin,
+                            is_loaded: lint.is_loaded,
                             depr: None,
                         })
                         .lint_ids
@@ -249,7 +247,7 @@ impl LintStore {
             alias,
             LintGroup {
                 lint_ids: vec![],
-                from_plugin: false,
+                is_loaded: false,
                 depr: Some(LintAlias { name: lint_name, silent: true }),
             },
         );
@@ -257,21 +255,21 @@ impl LintStore {
 
     pub fn register_group(
         &mut self,
-        from_plugin: bool,
+        is_loaded: bool,
         name: &'static str,
         deprecated_name: Option<&'static str>,
         to: Vec<LintId>,
     ) {
         let new = self
             .lint_groups
-            .insert(name, LintGroup { lint_ids: to, from_plugin, depr: None })
+            .insert(name, LintGroup { lint_ids: to, is_loaded, depr: None })
             .is_none();
         if let Some(deprecated) = deprecated_name {
             self.lint_groups.insert(
                 deprecated,
                 LintGroup {
                     lint_ids: vec![],
-                    from_plugin,
+                    is_loaded,
                     depr: Some(LintAlias { name, silent: false }),
                 },
             );