Differentiate between modules and plugins

Differentiate between modules and plugins

Now that modules can load (multiple) plugins dynamically, we really need
to start differentiating between modules and plugins. Most of this
differentiation needs to be only exposed to PluginLoader, rest of the code
just deals with plugins mostly. Another implication is the change in module
interface: function that each module needs to provide is "module_init" rather
than "load_plugin" now.