remove: some logging from Transaction.populate_rpm_ts().

It is not used to debug anything and by error ends up in stdout on CLI runs
with '-v'.
doc: api_base.rst pointing to nonexistent method.
refactor: turn dnf.conf into a package.
refactor: move yumvar out into its proper module dnf.conf.substitutions.

Related: RhBug:1104757
move: rpmUtils -> rpm.

rpmUtils is a weird name. Also non-PEPy. These things are important now
when dnf.rpm.detect_releasever() is about to be exposed in the API.
api: document client's responsibility to preset the substitutions. (RhBug:1104757)