Move the explain internal package to the new compass-explain repo.

• Move all code from internal-packages/explain to 10gen/compass-explain
• Use the khaos template to generate the plugin and make necessary changes to the template if problems arise.
• Move all Validation tests from Compass to the compass-explain repo
• Add compass-explain to the compass-lite and compass-enterprise distribution configs in the package.json in Compass (for now)
• Remove internal-packages/explain