Improve fetch_for_repository performance

[jacobobq]

Calling Resource#repository produces a call to safe_constantize on the repository_raw string. Such calls are expensive because they trigger a loading cascade of other constants. In this case, it's also unnecessary since you can find repositories by comparing the strings directly, so we bypass the need for it completely.

We've also made the method more scalable for successive calls by creating an appropriate hash index for direct repository_raw lookups.

[gtrias]

Amazing!