Add infrastructure to allow linking static qt plugins into static qml plugins

The motivating case is linking qwebp plugin into ios quickcontrols style plugin.

https://codereview.qt-project.org/c/qt/qtdeclarative/+/444609/4..5/src/quickcontrols2/ios/CMakeLists.txt#b116

What needs to happen is that
1) linking works
2) cmake package for ios qqc2 style creates a find_package dependency on the webp plugin
3) qmake prl and pri files don't contain hardcoded absolute paths, but use relative paths