hadrian: handle findExecutable "" gracefully

hadrian may invoke findExecutable "" at run-time due to a certain
program is not found by configure script. Which is fine and
findExecutable is supposed to return Nothing in this case. However, on
Windows there's a directory bug that throws an exception (see
https://github.com/haskell/directory/issues/180), so we might as well
use a wrapper for findExecutable and handle exceptions gracefully.

(cherry picked from commit 1e5752f6)
(cherry picked from commit 78150bfc)

hadrian/src/Hadrian/Utilities.hs

@@ -19,6 +19,7 @@ module Hadrian.Utilities (
     copyFile, copyFileUntracked, createFileLink, fixFile,
     makeExecutable, moveFile, removeFile, createDirectory, copyDirectory,
     moveDirectory, removeDirectory, removeFile_, writeFileChangedBS,
+    findExecutable,
 
     -- * Diagnostic info
     Colour (..), ANSIColour (..), putColoured, shouldUseColor,
@@ -668,3 +669,7 @@ renderUnicorn ls =
     ponyPadding = "                                            "
     boxLines :: [String]
     boxLines = ["", "", ""] ++ (lines . renderBox $ ls)
+
+-- Workaround for https://github.com/haskell/directory/issues/180
+findExecutable :: String -> IO (Maybe FilePath)
+findExecutable exe = IO.catch (IO.findExecutable exe) $ \(_ :: IO.IOException) -> pure Nothing

hadrian/src/Rules/Docspec.hs

@@ -2,8 +2,6 @@ module Rules.Docspec
   ( docspecRules
   ) where
 
-import System.Directory (findExecutable)
-
 import Base
 import Context.Path
 import Settings.Builders.Common

hadrian/src/Rules/Lint.hs

@@ -4,7 +4,6 @@ module Rules.Lint
 
 import Base
 import Settings.Builders.Common
-import System.Directory (findExecutable)
 import System.Exit (exitFailure)
 
 lintRules :: Rules ()

hadrian/src/Settings/Builders/RunTest.hs

@@ -16,7 +16,6 @@ import Settings.Builders.Common
 import qualified Data.Set    as Set
 import Flavour
 import qualified Context.Type as C
-import System.Directory (findExecutable)
 import Settings.Program
 import qualified Context.Type