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/command_callback/test_haskell_ghc_mod_command_callbacks.vader

@@ -0,0 +1,10 @@
+Before:
+  call ale#assert#SetUpLinterTest('haskell', 'ghc_mod')
+
+After:
+  call ale#assert#TearDownLinterTest()
+
+Execute(Default should use ghc-mod):
+  AssertLinter
+  \ 'ghc-mod',
+  \ ale#Escape('ghc-mod') . ' --map-file %s=%t check %s'

test/command_callback/test_haskell_hlint_command_callbacks.vader

@@ -1,5 +1,9 @@
 Before:
   call ale#assert#SetUpLinterTest('haskell', 'hlint')
+
+  let g:ale_haskell_hlint_executable = 'hlint'
+  let g:ale_haskell_hlint_options = ''
+
   let b:base_opts = '--color=never --json -'
 
 After: