plugins: Introduce generated, static core plugin registry (#54118)

* Refactor towards template/codegen framework

* Add templates for plugin gen

* Add Go codegen for plugins; overhaul framework, too

* Add new codegen output; assorted framework fixes

* Regenerate after merge

* Remove accidental commit file, update templates

* Export the pfs.Tree loader from plugin types

* Print details from cuetsy errors

* Generate loaders for all plugins and list in registry

* Use pfs_gen.go over lineage_gen.go

* Un-un-ignore main file

* Introduce simple List static registry for plugins

* Last tweaks to codegen

* remove unused tvars

* Ensure loop-local instances for both vars

* Generate pfs parsing in-place in registry

* Stop generating pfs_gen.go

* Move Tree into pfs, rename subdir

* Change package name to match dir

* Ignore gocyclo on HTTPServer.getNavTree

pkg/framework/coremodel/registry/provide.go

@@ -54,6 +54,6 @@ func provideBase(lib *thema.Library) *Base {
 //
 // The returned slice is sorted lexicographically by coremodel name. It should
 // not be modified.
-func (s *Base) All() []coremodel.Interface {
-	return s.all
+func (b *Base) All() []coremodel.Interface {
+	return b.all
 }