This produces a directory following the Angular Package layout spec.
Includes integration test coverage by making a minimal ng_package in integration/bazel.
Unit tests verify the content of the @angular/core and @angular/common packages.
This doesn't totally match our current output, but is good enough to unblock some
early adopters.
It re-uses logic from the rollup_bundle rule in rules_nodejs. It should also
eventually have the .pack and .publish secondary targets like npm_package rule.
PR Close#22221
Alex Eagle