Resolve map references (`mapref`)

The mapref step resolves references from one DITA map to another. This step is implemented in XSLT.

Maps reference other maps by using the following sorts of markup:

As a result of the mapref step, the element that references another map is replaced by the topic references from the other map. Relationship tables are pulled into the referencing map as a child of the root element (<map> or a specialization of <map>).