As per discussion at the KF6 sprint, see also https://phabricator.kde.org/T14233.
This is needed to support both Qt 5 and Qt 6 from the same ECM codebase, and is generally cleaner as random includes doing find_package calls can have interesting and unexpected side effects.
However, this will essentially break all current users (once they bump their ECM requirement), as we tend to include ECM modules before finding Qt. And this module is used indirectly by things like the default install location module, which we include practically everywhere. So we probably want to plan and coordinate the rollout of this a bit. On the upside, this will not break silently.
There are more places in ECM with the same problem, which would then need a similar change still.