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.
Fix #82: Only include Scala.js-emitted .js files in packageJS.

Other .js files on the classpath are not bundled by default by
packageJS (nor optimizeJS). Of course, .js files added explicitly
to (sources in (Compile, packageJS)) are still packaged, e.g.,
the typical startup.js file.
Merge pull request #299 from sjrd/packagejs-only-scalajs-files

Fix #82: Only include Scala.js-emitted .js files in packageJS.