Assign navtitle property on imported file

When Antora resolves page references from a nav file, it looks for the asciidoc.navtitle property on the referenced file to use as the implicit text. When Atlas imports files from the manifest, it does not set this property. Ergo, the text of the references falls back to the macro's target (i.e., the resource reference).

Atlas should ensure that the doctitle, xreftext, and navtitle are all exported and imported (however it is done) to ensure that these properties on the imported file return the same value as they would when not imported. In other words, the text of the xref shouldn't change when using Atlas.