composer.json: fix compatibility with dg/dibi requirements

This fixes compatibility which you have broken in 326376159f62e70c85b182a3d8e17b3f4588dbab. Now when you depend on two libraries and one of them depends on `dg/dibi` and the other one on `dibi/dibi`, only `dibi/dibi` will be installed.

Note: This should be tagged 2.1.1.