Support ij1.plugin.dirs also in modern mode

Fiji offers that feature that ImageJ 1.x plugins are also discovered in directories listed in the `ij1.plugin.dirs` system property (or, if unset, in `$HOME/.plugins/`). Let's support that for ImageJ2, too. Incidentally, this will help with changing the legacy startup as we are about to do in imagej-legacy, where we reuse the same class loader for ImageJ 1.x, ImageJ 2 and for the plugins.