Allow annotations to inherit across namespaces

Problem/Motivation

While working on #2560795: Source plugins have a hidden dependency on migrate_drupal, I discovered (with help from @chx and @tim.plunkett) that it's impossible (at least without doing a bunch of black magic) for an annotation class to extend another annotation class if they reside in different namespaces. This is a major gotcha for anyone who wants to define a new annotation by extending an existing one.

Proposed Resolution

AnnotatedClassDiscovery should accept an array of additional namespaces in which to search for annotation classes.

Remaining Tasks

^^ Write that, in patch form. Commit heartily, then sit back and have a drink.

API Changes

Additional argument to AnnotatedClassDiscovery::__construct() and greater flexibility in defining annotations.