Add better support for Haskell stack compiler tools (#1851)

* Add better support for Haskell stack compiler tools

This commit adds support for `stack` as the executable of a tool. This
follows a pattern that has been implemented for `bundler`'s tool chain.

* Move hlint command to linter file
* Add vader test for stack exec handling
* Update ghc-mod to support stack execution

`ghc-mod` was previously broken into 2 linters.

1. ghc_mod
2. stack_ghc_mod

This additional linter is not necessary with proper support for
executable variables and `stack exec` handling.

* Support stack exec in hfmt
* Support stack in hdevtools

test/fixers/test_hlint_fixer_callback.vader

@@ -1,9 +1,4 @@
 Before:
-  Save g:ale_haskell_hlint_executable
-
-  " Use an invalid global executable, so we don't match it.
-  let g:ale_haskell_hlint_executable = 'xxxinvalid'
-
   call ale#test#SetDirectory('/testplugin/test/fixers')
 
 After:
@@ -17,7 +12,7 @@ Execute(The hlint callback should return the correct default values):
   AssertEqual
   \ {
   \   'read_temporary_file': 1,
-  \   'command': ale#Escape('xxxinvalid')
+  \   'command': ale#Escape('hlint')
   \     . ' --refactor'
   \     . ' --refactor-options="--inplace"'
   \     . ' %t',