Handle bare .class plugins in ij1.plugin.dirs

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Avoid putting plugins from ij1.plugin.dirs into submenus

ImageJ 1.x does not remember the directory hierarchy when parsing
for plugins; instead, it reconstructs it after the fact from looking
at the absolute path name and stripping off the prefix obtained from
the system property plugin.dir. This wreaks havoc with our support
for ij1.plugin.dirs because those directories most certainly do not
have the value of plugin.dir as prefix.

So let's just skip the complete submenu determination when handling
plugins from ij1.plugin.dirs.

Together with the previous commit (to support bare .class plugins in
ij1.plugins.dir), this addresses https://github.com/fiji/fiji/issues/48.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>