rename: Base.activate_sack() -> Base.fill_sack().

More intuitive, sounds less technical.
rename: Base.build_transaction() -> Base.resolve()

More intuitive.
refactor: Comps.*iter methods are not properties.

Methods with iterators for return values are poor candidates for properties,
iterators are by nature quite mutable.
doc: delete the obsoleted API docs
doc: foundations of the API documentation.
Officially call dnf.yum.config.YumConfig just dnf.conf.Conf.