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/plugins/pfs/errors.go

@@ -17,6 +17,9 @@ var ErrInvalidRootFile = errors.New("plugin.json is invalid")
 //   - A required slot for its type is not implemented (e.g. panel plugin does not implemented Panel)
 var ErrImplementedSlots = errors.New("slot implementation not allowed for this plugin type")
 
+// ErrInvalidCUE indicates that a plugin's model.cue file contained invalid CUE.
+var ErrInvalidCUE = errors.New("CUE syntax error")
+
 // ErrInvalidLineage indicates that the plugin contains an invalid lineage
 // declaration, according to Thema's validation rules in
 // ["github.com/grafana/thema".BindLineage].