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 rust-lang#29597.

Author: nnethercote

Cargo.lock

@@ -3666,7 +3666,6 @@ dependencies = [
  "rustc_monomorphize",
  "rustc_parse",
  "rustc_passes",
- "rustc_plugin_impl",
  "rustc_privacy",
  "rustc_query_system",
  "rustc_resolve",
@@ -3956,7 +3955,6 @@ dependencies = [
  "rustc_monomorphize",
  "rustc_parse",
  "rustc_passes",
- "rustc_plugin_impl",
  "rustc_privacy",
  "rustc_query_impl",
  "rustc_query_system",
@@ -4266,21 +4264,6 @@ dependencies = [
  "tracing",
 ]
 
-[[package]]
-name = "rustc_plugin_impl"
-version = "0.0.0"
-dependencies = [
- "libloading 0.7.4",
- "rustc_ast",
- "rustc_errors",
- "rustc_fluent_macro",
- "rustc_lint",
- "rustc_macros",
- "rustc_metadata",
- "rustc_session",
- "rustc_span",
-]
-
 [[package]]
 name = "rustc_privacy"
 version = "0.0.0"

compiler/rustc_codegen_cranelift/scripts/test_rustc_tests.sh

@@ -76,7 +76,6 @@ rm -r tests/run-make/split-debuginfo # same
 rm -r tests/run-make/symbols-include-type-name # --emit=asm not supported
 rm -r tests/run-make/target-specs # i686 not supported by Cranelift
 rm -r tests/run-make/mismatching-target-triples # same
-rm -r tests/run-make/use-extern-for-plugins # same
 
 # requires LTO
 rm -r tests/run-make/cdylib