fix(injector): allow multiple loading of function modules

move the loadedModules lookup to be performed only on strings in order to allow loading functions that look the same. this issue caused only the first `angular.mock.module(object)` invocation to work since the loadModules lookup blocked all following invocations.

closes #7254
I have a PR pending that changes uids to simple number counters (#7759) but let's keep things as they are here, I'll rebase my branch