Plugin structure for chart/visualizations/expressions

[timroes]

After some discussion we figured, that we should strive to the following folder structure for visualization related plugins:

└ chart_expressions (just a folder, see 1)
  └ expression_tagcloud
  └ expression_pie
  └ ...
└ vis_types (just a folder, see 2)
  └ pie (previous vis_type_pie)
  └ tagcloud (previous vis_type_tagcloud)
  └ ...
└ visualize (the editor)
└ visualizations (the current visualizations plugin with shared visualize infrastructure)
└ charts (utility methods for all charts, e.g. palette service)

(1) This folder will contain all plugins offering a chart function and it's corresponding renderer. They are meant to be used among other plugins, e.g. every renderer/chart function that is supposed to be used shared between Visualize, Lens or Canvas should be extracted to here. We originally discussed having only one plugin for all chart_expressions. We decided against this, since we expect there do be different owners of those, and thus separate plugins make it easier to handle that.

(2) This folder will contain the legacy visualization types (i.e. visualize visualizations). If their renderer/expression is not shared with any other plugin, it can be contained within the vis_type_* plugin in this folder. If it's sharing a renderer/expression with Lens or Canvas, the renderer must be extracted into the chart_expression folder (see 1). When moving folders here we will strip of the vis_type_ of the folder name since it's kind of redundant. It should nevertheless remain in the plugin's id.

[elasticmachine]

Pinging @elastic/kibana-app (Team:KibanaApp)

[timroes]

cc @ppisljar (since we discussed this)
cc @clintandrewhall I think it makes sense that we would group all your expression_* plugins also under the chart_expression folder in this case

[stratoula]

This has been addressed!