build(single-spa-angular): move internal functions to single-spa-angular/internals package

[arturovt]

Internal functions are moved into a separate package. This will be possible to re-use them when implementing additional packages for @angular/elements and renderComponent.

[arturovt]

babel/babel-eslint#681

[joeldenning]

Interesting, so the idea is that the single-spa-angular package would have the ability to mount/unmount an Angular module, Angular component, or Angular element?

Why create separate npm packages for this instead of just separate files within the same package? You can do import 'single-spa-angular/internal' without internal having its own package.json. Is making a separate package.json required for ng-packagr to handle import paths within a package?

[arturovt]

It's not an npm package. In theory, it's called "secondary entry point". Here is very short docs: https://github.com/ng-packagr/ng-packagr/blob/master/docs/secondary-entrypoints.md

So we gonna have a single package single-spa-angular and sub-packages single-spa-angular/{elements,render-component}. Thus people will not bundle the functionality they don't need or use. If the developer wants to bootstrap/mount/unmount only Angular module then he will use single-spa-angular, if he wants to bootstrap an Angular element module that will define custom web component then he will use single-spa-angular/elements and etc.

Everything will be inside single-spa-angular. By creating sub-packages we reduce the tight coupling and add the ability to import only that stuff that you wanna use.

[joeldenning]

Looks good to me