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.
So we expect all local paths to be loaded with `./`?

Eg:

<script type="module">
  import "./app.js"
</script>

works by requesting relative to the baseURL, but `app.js` does not?