Improve import action of hsimport

[fendor]

Current implementation of the hsimport plugin can not deal with constructors, e.g. trying to import Null from Data.Aeson will wrongly try to import Data.Aeson (Null) instead of the correct Data.Aeson (Value(Null)). This pr aims to change that, while also preparing the implementation of #1191.
Clones the API of HsImport with small adaptations for our use case, e.g. currently no support for qualified imports.
This pr also fixes the import of operators.

• Tests
• Cleanup code

[lorenzo]

Looks great!

[fendor]

Example of what will be possible now:

{-# LANGUAGE NoImplicitPrelude #-}
import           System.IO (IO)
-- | Main entry point to the program
main :: IO ()
main =
    when True
        $ hPutStrLn stdout
        $ fromMaybe "Good night, World!" (Just "Hello, World!")

Only using Code Actions, this can be turned into:

{-# LANGUAGE NoImplicitPrelude #-}
import           System.IO (IO, hPutStrLn, stdout)
import           Prelude (Bool(..))
import           Control.Monad (when)
import           Data.Maybe (fromMaybe, Maybe(Just))
import           Data.Function (($))
-- | Main entry point to the program
main :: IO ()
main =
    when True
        $ hPutStrLn stdout
        $ fromMaybe "Good night, World!" (Just "Hello, World!")

Of course, this will be documented via a test.

[fendor]

@bubba, would you mind taking a look at the tests again? I can run them, but I dont know how to solve the issue. The test case worked when executing each code action manually.

[lukel97]

@fendor sure thing! This looks really cool, so I was planning on trying it out anyway :)

[fendor]

Currently, it seems like import actions for tpyes do not work at all. I would like to take a look at it, before this is merged.