Lots of sub- and sub-subcommands are derived from the base class of
the invoking command, notably below cmds.projects. That provides some
properties shared across the ancestor hierarchy of a command, but is
semantically unsound. Introduce jw.pkg.BaseCmd class as a place to
provide basic helpers shared across all commands used in a
jw.pkg.App's context. Also add cmds.projects.Cmd to be used by other
commands in cmds.projects in later commits.
Signed-off-by: Jan Lindemann <jan@janware.com>
