Get rid of load.js and module utils

[Reinmar]

This has been discussed in #349 (comment).

(...) then we could totally remove the load.js stuff and the whole module.js. They only make sense in the source mode, but then you can easily use imports and you don't need loading features by strings. Side note: We have to define anyway how can you instantiate two editors with a different set of features, when using one bundle. Related tickets: ckeditor/ckeditor5-dev-bundler-rollup#1 and ckeditor/ckeditor5-core#28 and ckeditor/ckeditor5-core#27 (comment).

The point is that we implemented load.js to match the functionality of the previous approach, but it turns out that it can't really be used and, also, it isn't really needed.

In the source version of editor you have all the modules as separate files and you can import what you need and load things by config.features = [ FeatureA, FeatureB ];. In the compiled version loading by string doesn't work because all the code is bundled into one blob and the mapping between names and constructors is being lost (in the source version this mapping is represented by the file structure).

Finally, there are evidences that we'll need to name features anyway and compile the name<->constructor mapping into the editor (as a config most likely).