Extend the qtdoc generator to handle module pages generated by qdoc

Some of the Qt functions and enums are accessible by including a header, such as the [<QtMath>|https://doc.qt.io/qt-5/qtmath.html], [<QtAlgorithm>|https://doc.qt.io/qt-5/qtalgorithm.html], and so on. Typically, such functions, enums, and typedefs are documented using the \headerfile command in qdoc. 

But in Qt for Python some of these functions are available with just the module import. For example, importing PySide2.QtCore should provide access to a few QtMath functions.  We could tell qdoc to associate these functions with a module instead of headerfile, so the functions are listed in the <module_name>-module.webxml files. Now, the qtdoc generator should parse this info. in the module webxmls and append it to the module index pages it generates.