traceur.get to use simplified names

This allows for code like `require('traceur').get('codegeneration/ScopeTransformer')`.

This changed with the module name changes, as module names contain extensions, but it can still make sense to consider `traceur.get` as contained within a cleaner namespace.
This function is emulating `require()`: it synchronously returns a module-relative name computed at runtime.  I think the argument should be required to be relative and similar to the moduleName.  

I think we should also consider not supporting it. It creates the impression that ES6 will support `require()` like semantics.  The function will return `undefined` when users expect a file to be read.  We should guide users to use `import` or `System.import()`.