Drop autoexporter usage

Problem: `autoexporter` is used in one place to re-export one module.
Probably one day it will re-export more, but at this point it's really
overkill.
Moreover, it causes Hackage build to fail for the reasons mentioned
here:
https://github.com/haskell/hackage-server/issues/821

Solution: stop using it, re-export manually.

package.yaml

@@ -50,9 +50,6 @@ default-extensions:
   - TypeApplications
   - TypeOperators
 
-build-tools:
-  - autoexporter
-
 ghc-options:
   - -Wall
   - -Wincomplete-record-updates

src/Xrefcheck/Scanners.hs

@@ -1,6 +1,10 @@
-{- SPDX-FileCopyrightText: 2018-2019 Serokell <https://serokell.io>
+{- SPDX-FileCopyrightText: 2018-2020 Serokell <https://serokell.io>
  -
  - SPDX-License-Identifier: MPL-2.0
  -}
 
-{-# OPTIONS_GHC -F -pgmF autoexporter #-}
+module Xrefcheck.Scanners
+  ( module Xrefcheck.Scanners.Markdown
+  ) where
+
+import Xrefcheck.Scanners.Markdown