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

* s/Declaration/Definition/g

* s/DeclForGen/DefForGen/g

* Rename some local vars

pkg/plugins/pfs/decl_parser.go

@@ -61,7 +61,7 @@ func (psr *declParser) Parse(root fs.FS) ([]*PluginDecl, error) {
 				Imports:         pp.CUEImports,
 				PluginMeta:      pp.Properties,
 				PluginPath:      path,
-				KindDecl:        kind.Decl(),
+				KindDecl:        kind.Def(),
 			})
 		}
 	}