doc: groom README and man page a bit.

Mainly, we are no longer in the 'experimental' phase.
doc: improve documentation on --cacheonly. (RhBug:1066743)
refactor: Move InfoCommand._print_packages to BaseCli.output_packages.
refactor: Move repo-pkgs info to a standalone class instead of reusing the InfoCommand.
refactor: Move MockBase methods to BaseStubMixin.
Fix "repo-pkgs info installed" behavior with respect to the documentation.
tests: Add tests of repo-pkgs info against the documentation.
refactor: Move repo-pkgs list to a standalone class instead of reusing the ListCommand.
refactor: Move repo-pkgs check-update to a standalone class instead of reusing the CheckUpdateCommand.
refactor: Move CheckUpdateCommand.check_updates to BaseCli.
refactor: Move repo-pkgs install to a standalone class instead of reusing the InstallCommand.