Only run stack if a stack.yaml config is found (#1752)

* Only run stack if a stack.yaml config is found

It is necessary to check for a stack.yaml file to distinguish between
cabal-only projects or stack projects (which are also cabal projects
since stack is built on top of cabal).

* Test that stack is called if stack.yaml exists

autoload/ale/handlers/haskell.vim

@@ -1,5 +1,15 @@
 " Author: w0rp <devw0rp@gmail.com>
 " Description: Error handling for the format GHC outputs.
+"
+function! ale#handlers#haskell#GetStackExecutable(bufnr) abort
+    if ale#path#FindNearestFile(a:bufnr, 'stack.yaml') isnot# ''
+        return 'stack'
+    endif
+
+    " if there is no stack.yaml file, we don't use stack even if it exists,
+    " so we return '', because executable('') apparently always fails
+    return ''
+endfunction
 
 " Remember the directory used for temporary files for Vim.
 let s:temp_dir = fnamemodify(ale#util#Tempname(), ':h')