dependencies: reject Qt6 pkg-config dependency if libexecdir is not known

This is needed for 6.1 support as noted in commit
a606ce22eb but at the time there was no
pkg-config detection method at all. The stub code for Qt6 didn't support
libexecdir at all, and the pkg-config files added to newer versions of
Qt6 didn't export that information either until a further bugfix release.

Enforce that this information is required.

mesonbuild/dependencies/qt.py

@@ -433,6 +433,13 @@ class Qt5PkgConfigDependency(QtPkgConfigDependency):
 
 class Qt6PkgConfigDependency(QtPkgConfigDependency):
 
+    def __init__(self, name: str, env: 'Environment', kwargs: T.Dict[str, T.Any]):
+        super().__init__(name, env, kwargs)
+        if not self.libexecdir:
+            mlog.debug(f'detected Qt6 {self.version} pkg-config dependency does not '
+                       'have proper tools support, ignoring')
+            self.is_found = False
+
     @staticmethod
     def get_pkgconfig_host_bins(core: PkgConfigDependency) -> str:
         return core.get_pkgconfig_variable('bindir', [], None)