Check public path for AMD modules

[jasongrout]

#3464 may need some more work. It seems that the webpack auto public path does not work inside of AMD modules, since by the time the AMD callback is called the currentScript is null. This would affect the various amd modules generated by the html manager. This is hinted at in #2191 (comment). See also #1652, #1650.

A workaround for automatically figuring out the public path in an AMD module using the require 'module' magic dependency is at jupyter-widgets/widget-cookiecutter#103

[jasongrout]

@xiaochen-db and I confirmed that using the html manager libembed-amd.js file from requirejs is currently broken on master. We do indeed need the fix from jupyter-widgets/widget-cookiecutter#103