Have "stack script" set import search path #3377

[mgsloan]

• Any changes that could be relevant to users have been recorded in the ChangeLog.md
• The documentation has been updated, if necessary.

This issue was brought to my attention by https://github.com/rcook . The primary problem is that stack scripts can only import modules if the script is run from the same directory as the modules. However, I realized that this implies a gnarlier issue, which is that since -i isn't being passed to runghc, it will use the work dir as a search dir for imports.

This also addresses part of the problem described in #3377 , which was a difference in how imports are treated when --compile is used.

Here's a concrete example. The contents of ~/tmp/hmm.hs is

#!/usr/bin/env stack
-- stack --resolver lts-13.0 script

import Yargh

main :: IO ()
main = yargh

The contents of ~/tmp/Yargh.hs is

module Yargh where

yargh :: IO ()
yargh = putStrLn "works!"

The contents of ~/Yargh.hs is

module Yargh where

yargh :: IO ()
yargh = putStrLn "YARGH, broken!

Check out what happens when I run the script from different directories:

mgsloan@treetop:~/tmp$ ./hmm.hs                                                                               
works!                                                                                                        
mgsloan@treetop:~/tmp$ cd ../                                                                                 
mgsloan@treetop:~$ ./tmp/hmm.hs                                                                               
YARGH, broken!      

This PR resolves the issue. After applying this patch, it behaves like this:

mgsloan@treetop:~/tmp$ ./hmm.hs                                                                               
works!                                                                                                        
mgsloan@treetop:~/tmp$ cd ../                                                                         
mgsloan@treetop:~$ ./tmp/hmm.hs                                                                               
works!                      

[rcook]

Thanks!