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>
Be more adamant about adding the plugins/ directory to the class path

EssentialLegacyHooks' missingSubdirs() might be called too early for
IJ.getDirectory("plugins"); to return anything but null...

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>
Verify that a bare HandleExtraFileTypes class can be found

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