Clean up data plugin

[lukeelmers]

Summary

Working to make a clear separation between shim code & "new platform" code, so that at the end of the migration process we can just delete the shims. Also trying to make everything consistent based on the most recent conventions we have had, so that the data plugin can serve as an example for future plugins and we can point other teams to it as the recommended way of doing things.

Proposed Conventions

These changes imply a few conventions I'd like to propose (which could ultimately be added to #39542 that @joshdover started, once we get a consensus here):

• Each plugin's server/ and public/ directory has:
  • index.ts which exports a function named plugin (already required by NP)
    • File must also export any public static code & types (already required by NP)
  • plugin.ts in a separate file which contains the plugin definition (already a recommended convention for NP).
    • This file would also export the type interfaces for Setup/Start contracts, named as NameSetup, and NameStart, and would define interfaces for it's own plugin dependencies, named as NameSetupPlugins and NameStartPlugins
    • I've updated this class to implement the proper Core-provided generic Plugin type as well
    • For clarity, class name is DataPublicPlugin (server would be DataServerPlugin). Stole this idea from @streamich 😉
  • setup.ts, which was originally added by @lizozom. This file would serve exclusively as a shim that gets imported by other plugins.
    • All shim code would live in this file, so that the plugin.ts file can remain a "clean" representation of how it will look in the new platform. In a perfect world, these shim files can simply be deleted when things move to new platform and everything Just Works(TM).
    • Other shim plugins import from here (inside their setup.ts shim files!) to get the setup contract for our shim plugin.
    • If the plugin ever introduces a start contract, a separate start.ts shim file would get created.
    • The shim file uses npSetup from ui/new_platform to inject core dependencies.

I'm working on a proposed set of conventions for static code & type exports, but the more I've experimented, the more trade-offs I run into. So for the sake of keeping this PR small (and not breaking a bunch of downstream imports), I'll branch that off into a separate PR & discussion thread.

Tasks

• Clean up plugin definition
• Move interpreter imports to the expressions service
• Create separate plugin.ts file and export a plugin function from index.ts, per recommended new platform conventions
• Remove extra static exports from plugin definition and move to index.ts
• Pass "real" core services in via shim so we can switch over to using them when needed
• [ ] Add shims for each individual service, since there will be legacy/shim code in those service definitions as well Not necessary yet as there aren't currently shim codes in top-level service definitions
• Improve organization of types & other static exports <-- will move this to a separate discussion/PR

[elasticmachine]

Pinging @elastic/kibana-app-arch

[elasticmachine]

💚 Build Succeeded

• continuous-integration/kibana-ci/pull-request

[lizozom]

In general, I love the separation between index \ plugin \ setup
I think that this should be applied immediately to the vis_markdown plugin as well, to see how it works.
And also to make sure that EPAM work with correct references.

[lizozom]

A bit unrelated but shouldn't we have registerFunction here?
I saw @ppisljar added it to expressions on markdown vis.

[lizozom]

This is great. :)

[lukeelmers]

Going to go ahead and close this now that we've aligned on various conventions -- we can do the cleanup over the course of other refactorings that are in progress