resourceloader: Move 'site' and 'user' logic to makeModuleResponse

resourceloader: Move 'site' and 'user' logic to makeModuleResponse

* Keep this out of makeLoaderImplementScript() to keep it more generic
  and to simplify future refactoring.

* Remove now-broken test case that asserted that the output varies
  by global debug mode.

* Make the test responsible for wrapping in XmlJsCode. Previously
  this magically happened because the module name was "user" in the
  last test case.

* Make makeLoaderImplementScript protected. It's not used anywhere
  outside ResourceLoader and we should keep it that way.

Test plan:
* Verify output unchanged:
  - load.php?modules=user&only=scripts&user=Admin (raw code)
  - load.php?modules=user&user=Admin (implement with unwrapped string)
  - load.php?modules=jquery.client (implement with closure)

Change-Id: I527d01926fb6e4ce68c931695d830cdb9ceb608c