scripts: west_commands: Add packages command

Add a new west command as helper for package management of Zephyr
and modules.

The first package manager to get supported is pip, where west projects
can list individual packages or requirements files in their module file.

A convenience --install argument is added to install the packages instead.

Signed-off-by: Pieter De Gendt <pieter.degendt@basalte.be>