Use export import and .d.ts rewriting to consolidate module exports

[JoshuaKGoldberg]

Don't have things like...

import { IPreThing } from "IPreThing";
import { IMapsCreatrSettings } from "IMapsCreatr";

Instead:

1. Use export import in the main "MapsCreatr" module so consumers will just import from there:

   import { IPreThing, IMapsCreatrSettings } from "MapsCreatr";

2. Rewrite output .js and .d.ts files to not include require modules that have nothing in them. This way there isn't even an "IPreThing" distributed.

As a side bonus, the .d.ts files should have their type /// <reference path="..." />s rewritten to match the typings style.

[JoshuaKGoldberg]

Thought: it might be possible to rewrite files into the traditional namespace { ... } equivalents based on their folder structure.