Safeguard against IJ.getDirectory("plugins") returning null

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Reconcile class loader initialization

In ImageJ 1.x, there are always multiple code paths doing the same things.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Always initialize plugin class loader to see the bare .class plugins

This should address the issue that a bare HandleExtraFileTypes.class
in plugins/ was no longer seen, as reported by Davíð Þór Bragason.

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