list: new applet

The list applet provides a convenient way of inspecting both the available
and installed package databases by listing their contents.  In some ways,
it is similar to `apk search` but is considered to be a superset of
`apk search` functionality.

A few `apk list` criterion are not yet ready though, such as `apk list --depends`
which searches by runtime dependency (replacing `apk info --rdepends`).