qt5-static: More cmake-static work (WIP)

Plugin .prl files are read and the (linker) flags specified in
QMAKE_PRL_LIBS_FOR_CMAKE are added to INTERFACE_LINK_LIBRARIES
as is the full path for the static library that is each plugin

To complete this task we need to create the exe_plugin_import.cpp
file and add it to the executable target. The contents need to be:

// This file is autogenerated by cmake. It imports static plugin classes for
// static plugins specified using QTPLUGIN and QT_PLUGIN_CLASS.<plugin> variables.
Q_IMPORT_PLUGIN(QWindowsIntegrationPlugin)
Q_IMPORT_PLUGIN(QDDSPlugin)
..
..
Q_IMPORT_PLUGIN(QLastPlugin)