refact(plugins): each plugin in a separate pkg

[rafaelrubbioli]

For better readability and usage, each plugin is now in a subpackage of the plugin pkg.

closes https://github.com/tfaughnan/artoo/issues/4

[tfaughnan]

This looks good. Can you also rename the pattern and handler for each of the two plugins to just be Pattern and Handler?

E.g.echo.EchoPattern and echo.EchoHandler -> echo.Pattern and echo.Handler

[rafaelrubbioli]

@tfaughnan sure! do you also think is a good idea to rename internal names such as openaiRespPayload to respPayload?

[tfaughnan]

Yes those should be renamed to remove the redundant prefix -- good catch.

[rafaelrubbioli]

@tfaughnan done!

[tfaughnan]

LGTM, thanks!