Make widget managers completely in charge of loading widgets

[jasongrout]

This is an iteration on fixing #1242.

The basic idea is that we now consider the module/version to be semantic namespace (i.e., the spec for the model or view implementation), and the package/version to be a concrete implementation that can be loaded. The original thought was that if the package/version was not specified, it would default to the module/version. However, since package/version will be set on basic widgets classes, they must be overridden in custom widgets to be consistent.

[jasongrout]

However, since package/version will be set on basic widgets classes, they must be overridden in custom widgets to be consistent.

Most custom widgets inherit from DOMWidget or just Widget. A hack to get around the above issue would be to just give the package on the core widgets. It's not quite clear where it puts Widget, DOMWidget, and LabeledDOMWidget.

[jasongrout]

Okay, new thought: no package name stuff. The widget manager is responsible for loading correct versions of the models/views, full stop. Right now I'm thinking of three widget managers:

1. The notebook - widget libraries are installed and those are the ones available. It's up to the user to make sure the correct version of the library is installed.
2. JupyterLab - again, specific widget extensions are registered. It's up to the user to make sure the correct version of the library is installed.
3. The embedded widget manager - this is the only one that really uses the module_version as an npm package version. For many widgets, this will be correct. Any where it's not, the widget manager is responsible for translating module versions to package versions.

For each of these cases, the version of jupyter-js-widgets is special-cased to be the one we are extending.

So I'll strip this PR down to some of the cleanup I've done, and then the major change will be:

1. Make each widget manager completely in charge of loading the correct class for views and models. The default implementation will not assume require. I'm still undecided if the manager-base will provide code to load widgets from the jupyter-js-widgets package (leaning against it so that in the future, it's easier to separate out the base manager from our core widget implementation).
2. Change the documentation of the _module_* attributes to not refer to npm packages, but rather versions of the models/views (which can coincide with npm package names/versions by convention/coincidence).

[maartenbreddels]

If that means that views will not be loaded via require by default (in the notebook) I think that is a big plus for development, since that makes js 'reloading' via actually work, like what i do here.

[jasongrout]

Views would still be loaded using require -that's how things are done in the classic notebook

[jasongrout]

@SylvainCorlay, @maartenbreddels - Any thoughts on this before merging?