Support bare HandleExtraFileTypes.class again

Davíð Þór Bragason reported [to the mailing list](http://imagej.1557.x6.nabble.com/Handle-Extra-Files-Types-not-working-td5008224.html) that bare `HandleExtraFileTypes.class` files are no longer picked up. This was a design mistake in the way `noPluginClassLoader()` worked, and easily fixed.