plugindef: Move pluginmeta out of coremodels as standalone thema lineage (#56765)

* Get pluginmeta mostly moved over to pkg/plugins/plugindef

* Remove dead func

* Fix up pfs, use sync.Once in plugindef

* Update to latest thema

* Chase Endec->Codec conversion in Thema

* Comments on slash header gen; use ToSlash

* Also generate JSON schema for plugindef

* Generate JSON Schema as well

* Fix slot loading from kindsys cue decls

* Remove unused vars

* skip generating plugin.schema.json for now

Co-authored-by: Marcus Efraimsson <marcus.efraimsson@gmail.com>

pkg/codegen/coremodel.go

@@ -237,7 +237,7 @@ type tplVars struct {
 }
 
 func (cd *CoremodelDeclaration) GenerateTypescriptCoremodel() (*tsast.File, error) {
-	schv := thema.SchemaP(cd.Lineage, thema.LatestVersion(cd.Lineage)).UnwrapCUE()
+	schv := cd.Lineage.Latest().Underlying()
 
 	tf, err := cuetsy.GenerateAST(schv, cuetsy.Config{
 		Export: true,