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

ale_linters/haskell/stack_ghc.vim

@@ -5,7 +5,7 @@ call ale#linter#Define('haskell', {
 \   'name': 'stack_ghc',
 \   'aliases': ['stack-ghc'],
 \   'output_stream': 'stderr',
-\   'executable': 'stack',
+\   'executable_callback': 'ale#handlers#haskell#GetStackExecutable',
 \   'command': 'stack ghc -- -fno-code -v0 %t',
 \   'callback': 'ale#handlers#haskell#HandleGHCFormat',
 \})