[Interest] Refusing to generate non-lowercase name "A" for unknown foreign type

[Ulf Hermann]

> Unfortunately this is not enough in my real case, where `A` is not only 
> a different library, but is provided by a separate (debian) package,
> so `A` in the current project is an INTERFACE target, which 
> `qt_extract_metatypes()` falls over with an obscure error.

Indeed without metatypes we cannot generate any qmltypes. The 
metatypes.json files tell us all the interesting things moc has figured 
out about the library in question. The authors of the upstream library 
should make sure the metatypes are extracted, and there should be a 
standard location for them to be installed in Debian. I would guess that 
the metatypes for QtQuick, for example, are already installed somewhere. 
Otherwise pretty much nothing would work.

best regards,
Ulf