Rework referrer logic to be independent of traceur directory structure.


The overall output of the compiler in multi-file mode is to create a set of
modules in an output file with names beginning with a versioned package name.
The referrer option is encoding both the package name and the path to the
output file from the root of the package directory. EG traceur@0.0.67/bin.
The source is relative to the output directory (eg ../src/Compiler.js).
The referrer causes these relative names to normalize to package names,
eg traceur@0.0.67/src/Compiler. In locate() we map these names back to
the file system using the uncommon part of the referrer and the output
directory (which is the baseURL).
maybe use slice instead of substring?