Fix unable lookup source

[golonikum]

Fixes #1014

[coreyfarrell]

@golonikum thanks for the contribution. I've updated the tests to work with the corrected results and I made a small tweak to your code to use a variable which had already resolved the filename.

@bcoe @JaKXz I didn't remove it but I think the relFile passed to the transform function is unused. Possibly a remnant from days past, maybe from before caching-transform was split into a separate module.

[coveralls]

Coverage remained the same at 94.046% when pulling b535b37 on golonikum:fix-unable-lookup-source into a161d23 on istanbuljs:master.