Avoid parsing ij.jar for plugins

Now that we allow adding files to the class path of the plugin class
loader, we need to be careful to skip known non-plugin .jar files, to
avoid the dreaded 'duplicate plugins' warning.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Also parse unpacked plugin .jar files *without* plugins.config

Sometimes it is quite handy to be able to run unpacked plugins lacking
a proper plugins.config, e.g. when debugging. So let's support that
use case, too.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Support adding a plugin class path given a ClassLoader

Sometimes you already have a class loader that knows where to find your
ImageJ 1.x plugins you want to run inside the legacy environment.

This commit adds LegacyEnvironment#addPluginClasspath(ClassLoader) to
support that use case (the plugin class loader has to load the plugin
classes separately from the given class loader, of course, so they know
about *this* legacy environment's ImageJ 1.x).

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Make sure that LegacyEnvironment#addPluginClasspath() works

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