libcomps: add dnf.comps (wrapper) and a basic UT.
libcomps: add conditional packages.

allowed by libcomps commit a42b9c7.
libcomps: tests: add sanity test for GH issue 12.

https://github.com/midnightercz/libcomps/issues/12
drop 'overwrite_groups' config option.

It does nothing.
libcomps: measuring the size of the comps objects.
libcomps: add Comps.groups_by_pattern().