Issue #2818689 by rjacobs, tstoeckler: Add ability to scan multiple stream wrappers when locting library source

Squashed commit of the following:

commit aed963858529e94dd7c317794a428ec926d9e080
Author: Ryan Jacobs <rjacobs@422459.no-reply.drupal.org>
Date:   Sat Dec 31 17:27:21 2016 -0600

    Issue #2818689: Add GlobalLocator integration test.

commit 6e0225737795ea0716202e246c0a037ec15ba854
Author: Ryan Jacobs <rjacobs@422459.no-reply.drupal.org>
Date:   Wed Dec 28 23:01:55 2016 -0600

    Issue #2818689: Add global locator.

commit 2af9f79423221b79d11de500db4607a229bc2526
Author: Ryan Jacobs <rjacobs@422459.no-reply.drupal.org>
Date:   Sun Dec 18 22:07:12 2016 -0600

    Issue #2818689: Add ChainLocator and refactor StreamLocator into UriLocator.