Support building packages with Conan which sets an install prefix different from the Qt install prefix

Apparently Conan specifies a random CMAKE_INSTALL_PREFIX when building a package.

So if a Conan recipe is used to build qtsvg for example, it breaks our assumption that Qt repos / modules are installed into the Qt installation prefix (we assume CMAKE_INSTALL_PREFIX is always the initial Qt installation prefix).

And thus many things fail (syncqt is not found, because we look for it in the qt install prefix), qdoc is not found, and many other things.

Partially this can be fixed by deriving the Qt install prefix dynamically from the location of the Qt6 Cmake package.

It is unclear that this will suffice in all situations, for example finding tools of repos by Conan.

Regardless of Conan, I believe this will need to work properly also for the case when somebody downloads and installs qt into different location than where it was built, and wants to build an addon module (CMAKE_INSTALL_PREFIX in such a case will 99.99999999999% be different from the original build machine).