Load shim from package name if no shim for moduleName.

This is a possible fix for jrburke/r.js#310

The idea is to try and get the the shim for a module based on the package name in the case that the module name includes `packageConfig.main`.

In the case that you have a package

      {
          "name": "backbone",
          "location": "jam/backbone",
          "main": "backbone.js"
      }

then it would try and get `shim['backbone']` if there was no `shim['backbone/backbone']`