`findExecutable` and `findFile` could be lazier

[sjakobi]

$ touch "file.txt"
$ stack ghci
...
GHCi, version 7.10.3: http://www.haskell.org/ghc/  :? for help
Ok, modules loaded: none.
λ> import System.Directory
λ> findFile ("." : undefined) "file.txt"
*** Exception: Prelude.undefined
λ> findFile ("." : []) "file.txt"
Just "./file.txt"

GHC-7.6 didn't use to have this problem but the behaviour changed with 7789d1c.