Use __import__ instead of imp.load_module

__import__ has an advantage over imp.load_module: It will load the plugin's
package too, letting it do it's imports using it's full path, so it doesn't
need to modify sys.path anymore. For instance:

  On nfe we changed:

    plugin_root = os.path.dirname(__file__)
    sys.path.append(plugin_root)
    from nfeui import NFeUI

  To just:

    from nfe.nfeui import NFeUI

Change-Id: I46003c39c9d037d5d123fee6fdf2b9cbaa725a65