Tamar Christina authored Oct 25, 2017 and Ben Gamari committed Oct 25, 2017

This fixes the mirror script so it correctly queries haskell.org and if
packages aren't found check repo.msys2.org.

Also the mirror functionality now generates the md5 hashes after a
mirror fetch that can be placed in the md5sums file.

Test Plan:
 mk/get-win32-tarballs.sh fetch mirror

 and ./validate

Reviewers: austin, bgamari

Reviewed By: bgamari

Subscribers: rwbarton, thomie

Differential Revision: https://phabricator.haskell.org/D4118