Simplify lazy loading in Scala.js env by using encodedName in .sjsinfo.

Instead of inferring the relative path of a Scala.js-emitted file
wrt to the directories in the class path, we simply store the
encoded name of the class in the .sjsinfo, and use that information.
It is simpler and more reliable.
Merge pull request #298 from sjrd/simplify-lazy-loading

Simplify lazy loading in Scala.js env by using encodedName in .sjsinfo.