Provide an API to augment the plugin classpath

It is often a quite terrible idea to configure a specific `LegacyEnvironment` by setting the `plugin.dir` system property: this just asks for race conditions all over the place.

Instead, let's just add some methods to be able to augment the plugin class path via methods in a just-created legacy environment (i.e. *before* running anything inside).

That also helps the use case of the `fiji.Debug` class where one wants to add a plugin to the particular ImageJ 1.x instance started to debug, but the plugin classes are available in both `target/classes/` and `target/<artifactId>-<version>.jar` (leading to that dreaded -- and in this case misguided -- *duplicate plugins* error).

While at it, fix the problem raised in fiji/fiji#48: plugins were not properly discovered from `ij1.plugin.dirs`.