Add fully qualified backslash to all simple class usages

[mvorisek]

extracted from #9481

let's land this in a separate PR and minimize future diffs/conflicts

[mvorisek]

@alecpl I have rebased this PR, can it be merged to prevent further merge conflicts?

[mvorisek]

@pabzm can you review/merge? I need this one for #9481.

[alecpl]

I think I would prefer a use statements instead. Or we should do something with the release-1.6 code. Backporting anything has become hard.

[mvorisek]

@alecpl thank you for the idea using import to minimize conflicts!

So I reworked this PR:
a) all non-intra project usages are newly with leading \ - this is how they should be imported once namespaced
b) all intra project usages in currently non-namespaced files are kept, but dummy use X as X; imports are added

After #9481 the dummy imports will be replaced with meaningful imports.

Code can stay as it is, so conflicts across release branches are minimal.

Leading \ for non-instra project usages is asserted by CI.