Feature: allowAppImports

TODO list items:

[x] filtering matching files out of classic build pipeline. May involve monkey-patching the appInstance's trees.app.
[x] teaching the splitter to emit imports for package-name-prefixed imports that match, like "my-app/lib/example1".
[x] teaching the splitter to emit imports for relative imports for files that match, like "../lib/example1"
[x] handling webpack externals so that when an auto-imported module imports a classically-built app module it doesn't get duplicated inside webpack. This is an extension to what we already do to support v2 addons using ember-provided features.
[x] make babel-loader config sensitive to the fact that app code gets the full app babel config (whereas dependencies get a streamlined, standardized config)
[x] make sure that the dynamic imports work as well as the static-imports test scenario

embroider-build / ember-auto-import