AutoGen: Retrieve Qt version from moc as fallback

Consider the case where the find_package call for QtCore is wrapped in a
function call. Then AutoGen cannot determine the Qt version, because it
only looks at variables and directory properties. The former don't leave
the function scope and the latter are not set by default.

As a fallback, locate the moc executable via its target and call it with
the --version argument to determine the Qt version.

Issue: #22028