Review design of CLI plugins

[viferga]

Respect to the PR: #298
We should take some considerations:

1. Should we use a different name for core_plugin? Like cli_plugin or cli_core_plugin?
2. We should load only the cli/core_plugin when we run metacallcli
3. Should we extend the execution path mechanism into plugin_extension? so instead of using an environment variable for locating the plugins, we can use our existing methodology for locating plugins? This will easily solve the point 2.

[rxbryan]

I have chosen to just add support for sub folders in plugin_extension #306