Initialize plugins for ghc-8.6

[judah]

ghc-8.6, when using the GHC API, requires that plugins be initialized explicitly for each module. (Previous versions of the API initialized them automatically).
See for example:
sol/doctest#224

The following file that uses ghc-typelits-knownnat can reproduce the problem. It builds fine with ghc-8.6, but fails to load in the indexer. I applied a patch on our internal codebase similar to the above doctest PR, but wasn't sure how to test it in this codebase given the external dependency requirement.

{-# OPTIONS_GHC -fplugin=GHC.TypeLits.KnownNat.Solver #-}
{-# LANGUAGE DataKinds #-}
{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE TypeOperators #-}
module Main (main) where

import Control.Exception (assert)
import Data.Proxy
import GHC.TypeLits

-- | Requires ghc-typelits-knownnat to solve its constraints:
plusOne :: forall n . KnownNat n => Proxy n -> Integer
plusOne _ = natVal (Proxy :: Proxy (n + 1))

two :: Integer
two = plusOne (Proxy :: Proxy 1)

main :: IO ()
main = assert (2 == two) $ return ()