Kindsys: Replace "Declaration" with "Definition" (#62515)

* s/Declaration/Definition/g

* s/DeclForGen/DefForGen/g

* Rename some local vars

pkg/plugins/pfs/decl.go

@@ -13,7 +13,7 @@ type PluginDecl struct {
 	Imports         []*ast.ImportSpec
 	PluginPath      string
 	PluginMeta      plugindef.PluginDef
-	KindDecl        kindsys.Decl[kindsys.ComposableProperties]
+	KindDecl        kindsys.Def[kindsys.ComposableProperties]
 }
 
 func EmptyPluginDecl(path string, meta plugindef.PluginDef) *PluginDecl {