Investigate declaring dependencies in plugin descriptors

At the moment plugins are quite fragile as they don't declare dependencies. If you disable a plugin or module that another depends on, no warns are given and there will more than likely be a crash when plugin is loaded.

If the plugin/module is diasabled after the depending plugin has already been loaded, the plugin should be invalidated. At the moment, if the plugin has already been loaded they will continue to work because the module is usually cached. See PortletModuleDesciptor.getModule().