todo.
Get SPI in better shape.

* Rename DDLParserPlugin to ParserPlugin.
* Add ParserPluginFactory that's a factory for ParserPlugin instances; each
  instance will need to be parameterized by the current environment. This is
  what is instantiated by Lookup.
* Extend NamedProvider; change getModuleName() to getName().
Add SHOW MODULES command.
Add dummy plugin.

The MyPlugin / MyFactory pair responds to "meep" as equivalent to SHOW INSTANCES.
Add 'MODULE <modulename>' command.
'meep;' command from example plugin works.