repos: skip_if_unavailable is True by default now. (RhBug:984483)
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().
tests: sort tests in LibcompsTest better.
libcomps: support Base.select_groups() through the new comps.
libcomps: rename: comps.returnGroups()->comps.groups_by_pattern().
comps: adapt Base.group_lists() to the new comps interface.
comps: implement Group.ui_name
comps: rename: group.langonly -> group.lang_only.
refactor: GroupsError and CompsException are now both CompsError
remove: dnf.yum.comps.

replaced by dnf.comps now.
doc: group commands.

Also drop dnf.yum.comps autodoc when the module is gone.
tests: there's no dnf.yum.comps any more.
tests: basic environemnt parsing works in libcomps now.