Discover plugins only once

[ctrueden]

Right now, every time a new Context is created, it calls its PluginIndex's discover() method again, which rediscovers all the plugins and adds them to the index again—without clearing out the previous plugins.

This is all rather silly. Let's change PluginIndex to call findPlugins on its given PluginFinder immediately upon construction. Then we don't need to keep a reference to the PluginFinder, and we can get rid of the obsolete discover() method.

Discovered by @Squareys (see knime-ip/knip-scijava#11).