Plugins in the plugin folder cannot be used if they are just .class files.

Some plugins are used by just putting a class file in the plugins folder. These plugins do not load properly in pyimagej.

To reproduce this, take an example plugin and compile it to a class file and place it in the fiji plugins folder.

```
import ij.plugin.PlugIn;

public class Example_Plugin implements PlugIn {
@Override
public void run(String s) {
System.out.println("That's the one");
}
}
```

Starting fiji, the `Example Plugin` will be available in the menu and running it should produce the desired output.

Starting pyimagej.

import imagej
ij = imagej.init("Fiji.app", mode='interactive')
ij.ui().showUI()

Note the "Fiji.app" is the path to the Fiji that has the plugin installed. This should display the UI with Example Plugin in the menu. Trying to run it will cause a class not found exception.

Reproduce the issue with a compiled .class plugin in the Fiji.app plugins folder, then initialize pyimagej in interactive mode and run Example Plugin. Trace the class-loading path that produces the ClassNotFoundException; done means the plugin appears in the menu and runs through pyimagej, printing the expected output.

Written by the indexing model from the issue text.