refactor: use click instead of docopt for CLI

In order to have command code and CLI in one place while preserving
history of apkg.lib:

* add new click CLI directly into apkg.lib.* modules
* remove old docopt CLI modules apkg.commands.* (for the transition)

Follow-up patch moves apkg.lib to apkg.commands finishing the merge
of apkg.lib (code) and apkg.commands (CLI) in a transparent manner.

This patch preserves the original CLI with minor enhancements.

The only incompatible CLI change is the new handling of log level:

    apkg -L verbose build

-L/--log-level choice option needs to be supplied before apkg (sub-)command.

Commands are now non-interactive by default with legacy -y/--yes option
being still supported but officially replaced by --ask/--no-ask which
allows change of default in the future without impacting CLI.

Fixes: #54