pkgix: Add support for overriding builtin commands with plugins

The builtin commands (e.g. install) can now be overridden by plugins.
This allows to implement additional functionality and more advanced
versions of these commands without make core pkgix more complex.

Signed-off-by: Marco Elver <me@marcoelver.com>
pkgix: suppress warning on arg_nodeps > 1

This can be used by plugins, to e.g. implement more advanced dependency
resolution.

Signed-off-by: Marco Elver <me@marcoelver.com>