Support plugin discovery in sub-directories

[joshdover]

In order to support a more organized directory structure within src/plugins and x-pack/plugins, we need to be able to discover and load plugins that are in subdirectories, not only the top-level.

Discovery algorithm should work something like this:

1. Scan current directory for kibana.json file
   • If kibana.json file is found, return the discovered plugin, do not scan subdirectories
   • If kibana.json file is not found, run step (1) on each subdirectory recursively

This should have no affect on the dependency mechanism of the Platform, just where plugins may live in the directory structure.

[elasticmachine]

Pinging @elastic/kibana-platform (Team:Platform)