Track extra-source-files of plugins more accurately (haskell#4105)

Co-authored-by: Michael Peyton Jones <me@michaelpj.com>

Author: fendor

haskell-language-server.cabal

@@ -22,10 +22,18 @@ extra-source-files:
   test/testdata/**/*.cabal
   test/testdata/**/*.yaml
   test/testdata/**/*.hs
+  test/testdata/**/*.json
 
+  -- some README's have a .gif showcasing the feature
+  plugins/**/*.md
+  plugins/**/*.gif
+
+  -- These globs should only match test/testdata
   plugins/**/*.project
+  plugins/**/*.expected
   plugins/**/*.cabal
   plugins/**/*.yaml
+  plugins/**/*.txt
   plugins/**/*.hs
 
   bindist/wrapper.in