urlCache should die in a fire

[unscriptable]

urlCache was added to solve some use cases that curl.js didn't handle. Unfortunately, it has caused other issues. I'm thinking it's really not the right solution. I'll collect the use cases for urlCache here, so we can figure out the best way to remove it.

What is urlCache? As of a few versions back, curl.js has two internal caches: one for modules keyed by module id (cache) and one for modules keyed by url (urlCache). If curl can't find a module in cache, it looks in urlCache.

If we remove urlCache, issue #260 will be fixed.