Figure out .prl file generation for top-level Qt builds

The change in https://codereview.qt-project.org/c/qt/qtbase/+/303811 disabled .prl file generation for static top-level Qt builds, because it broke configuration.

The underlying issue is that targets are used in generator expressions for the .prl files, but the targets are not in scope (PNG::PNG found in qtbase, but missing in qtsvg scope).

We need to find a way to fix this (if there is any).

EDIT: It seems it's a problem for shared top-level builds too.
After https://codereview.qt-project.org/c/qt/qtbase/+/305759 went it, top-level builds fail to configure on Linux, because OpenGL:GLX target can not be found while generating prl files for static internal modules in qttools repo.