If you wish to add a wic plugin in your own layer, wic expects it to be placed in scripts/lib/wic/plugins/source (or scripts/lib/wic/plugins/image for image plugins). This is inconsistent with devtool, recipetool and oe-selftest that look for extensions under lib/ in additional layers.
Thinking about this it should look in both places to avoid breaking existing layers, however where we document the path we should be specifying the lib/ one.
this should align the path for wic http://git.yoctoproject.org/cgit/cgit.cgi/poky/commit/?id=b5a3625e5934f3eabebbc2253765194e4ada2fa1