non-legacy namespace package

[bertsky]

The current legacy approach to namespace packaging does not work with development/editable installation. See OCR-D/ocrd_all#433

This PR fixes it for this package – analogous PRs to the other qurator repos will follow.

[mikegerber]

I'd prefer getting rid of the qurator namespace instead - e.g. just using "sbb_images", not "qurator.sbb_images" here. There really is no benefit in sharing the namespace, just problems.

[mikegerber]

(But it's up to @labusch and @cneud, just my opinion)

[bertsky]

I'd prefer getting rid of the qurator namespace instead - e.g. just using "sbb_images", not "qurator.sbb_images" here. There really is no benefit in sharing the namespace, just problems.

IIUC these problems will be gone for good with the new-style namespace packaging. (But don't hold my feet to the fire for it.)

A pro argument is: under the qurator namespace, you have full control over interdependencies – no clashes due to package names.