CRM-20600 - Angular deps - Allow declaration in PHP code

CRM-20600 - Angular deps - Allow declaration in PHP code

If third-parties are allowed to alter the HTML content, then they may
introduce new dependencies.  This means that they'll need to delcare those
dependencies.

Tangentially, this will also make it easier to construct more optimized
base-pages (e.g.  "make a base-page with module X plus all its
dependencies").