get categories from wikitext in the AddCategory bot

Using the textlib.getCategoryLinks() function instead of the
Page.categories() method, to prevent transcluded categories
from being added directly to the page content.

This follows I05fa90523f718f09de83d03aab3bd9d031e7cbbb

bug: 58084
Change-Id: I900de3d9d1f30cbf2034e7e14f7127e08644f29e