Versionless targets hide finalizers set on versioned targets

The logic in qt6_finalize_executable() iterates over the directly linked targets and looks at their INTERFACE_QT_EXECUTABLE_FINALIZERS target properties. When versionless targets are used, this doesn't work because the properties are set on the versioned targets. qt6_finalize_executable() should probably recurse to all targets as far as possible, not just rely on the directly linked targets.

Note that the versionless targets are created as INTERFACE IMPORTED rather than aliases, presumably because aliases weren't allowed to be non-global until CMake 3.18. This means we can't just test for an ALIASED_TARGET property.