Normalize all URLs to be as absolute as possible to avoid usage of 'util.relative'.

[loganfsmyth]

The consumer currently uses util.relative to take a URL and convert it to one that may be present in _sources. There are a few downsides to this approach:

• We have to normalize sources or else risk not finding the match
• We have to ensure that the sources conform to the expectation of relative URLs
• The usage of relative makes it unclear whether argument URLs are expected to be relative to the map, or the root, or either.
• We have to special-case 'sources' that just aren't relative, like sources: ['/a.js'] (which is also only special-cased on in sourceContentFor)

This patch centralizes all source lookups for the consumer in _findSourceIndex so that all APIs behave consistently. By converting all input paths to be as absolute as we can make them, we avoid the complexities of making URLs relative to one another, and it helps make clear how these behave.

[loganfsmyth]

Woops, didn't quite post what I meant to. Closing in favor of #366