disable auto js extensions

Here's a prototype for https://github.com/ModuleLoader/es6-module-loader/issues/218.

* In normalize, we normalize relative to the parentName, and when none is provided we normalize relative to the baseURL.
* Paths are applied in normalize, before normalization, only on names that do not start with "scheme:", "./", "../", "/".
* Locate does a final normalization relative to the baseURL

All tests passing.

Feedback welcome @caridy.
Actually this would mess up all uses of names like `@system`, `@traceur`. Or should these become schema names anyway?