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/codegen/tmpl/plugin_registry_ref.tmpl

@@ -0,0 +1,16 @@
+{{ template "autogen_header.tmpl" .Header -}}
+package registry
+
+import (
+	"github.com/grafana/grafana/pkg/plugins/pfs"
+	"github.com/grafana/thema"
+  {{ range .Plugins }}
+  {{ if .NoAlias }}{{ .PkgName }} {{end}}"{{ .Path }}"{{ end }}
+)
+
+func coreTreeLoaders() []func(*thema.Library) *pfs.Tree{
+  return []func(*thema.Library) *pfs.Tree{
+    {{- range .Plugins }}
+    {{ .PkgName }}.PluginTree,{{ end }}
+  }
+}