Have old API dox generation script cover all of language/

Summary:
Old script creates a tree from the Mainpage.dox files to
create a structure, and only covers the subdirs of the
Mainpage.dox files at the leaf nodes of the tree.
So with a Mainpage.dox file in language/{codegen,duchain,editor}
the files in the other subdirs below language/ will not be
covered, unless they also have a Mainpage.dox

Reviewers: KDevelop, kfunk

Reviewed By: KDevelop, kfunk

Subscribers: kfunk, kdevelop-devel

Differential Revision: https://phabricator.kde.org/D3762