Ensure that all Sass source URLs are canonical

[nex3]

See sass/sass#3908

[nex3]

Retargeting this as suggested in sass/sass#3908 (comment) to fix the loophole where the parser (and compile methods) may not canonicalize the source URLs they're passed, making it possible to have non-canonical URLs floating around. This is the root cause of non-absolute URLs showing up in the JS API.

I'm still marking this as blocked on 2.0.0, since it wreaks some havoc with the legacy JS API so we might as well wait until it's gone rather than devote a lot of effort trying to fix it.