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/tmpl/kind_corestructured.tmpl

@@ -15,7 +15,7 @@ const rootrel string = "kinds/structured/{{ .Meta.MachineName }}"
 // TODO standard generated docs
 type Kind struct {
 	lin    thema.ConvergentLineage[*{{ .Meta.Name }}]
-	jendec vmux.Endec
+	jcodec vmux.Codec
 	valmux vmux.ValueMux[*{{ .Meta.Name }}]
 	decl   kindsys.Decl[kindsys.CoreStructuredMeta]
 }
@@ -47,9 +47,9 @@ func NewKind(rt *thema.Runtime, opts ...thema.BindOption) (*Kind, error) {
 		return nil, err
 	}
 
-	k.jendec = vmux.NewJSONEndec("{{ .Meta.MachineName }}.json")
+	k.jcodec = vmux.NewJSONCodec("{{ .Meta.MachineName }}.json")
 	k.lin = tsch.ConvergentLineage()
-	k.valmux = vmux.NewValueMux(k.lin.TypedSchema(), k.jendec)
+	k.valmux = vmux.NewValueMux(k.lin.TypedSchema(), k.jcodec)
 	return k, nil
 }