Convert fixture themes to packages

spec/fixtures/packages/theme-with-package-file/package.json

@@ -1,3 +1,4 @@
 {
+  "theme": true,
   "stylesheets": ["first.css", "second.less", "last.css"]
-}
+}

spec/fixtures/packages/theme-with-ui-variables/package.json

@@ -1,3 +1,4 @@
 {
+  "theme": true,
   "stylesheets": ["editor.less"]
 }