feat(package): introduce a catch-all package angular.

modules/angular has no implementation, but depends on all the pieces
that make angular - core, di, directives, etc. It is the package that
all client apps will depend on.

modules/angular/src/angular.js

@@ -0,0 +1,6 @@
+/**
+ * Define public API for Angular here.
+ */
+export * from 'change_detection/change_detection';
+export * from 'core/core';
+export * from 'directives/directives';