Fixes #4890

Edward Z. Yang authored Oct 28, 2016 and Edward Z. Yang committed Oct 31, 2016

The key idea is that once we put PackageTests in its own package, we can
ensure that its Setup.hs is compiled with the same version of Cabal
library as package-tests is compiled with.  This means that LBI reading
will *always succeed* which is quite nice.

Signed-off-by: Edward Z. Yang <ezyang@cs.stanford.edu>

This allows specifying the main-is field as a C file.

This is closely based on patches by Edward Z. Yang, who in turn credits
and earlier set of patches by Irene Knapp. The slight difference in
this version of the patch is that it is adjusted to work with the new
approach where we have separate hs compile; c compile; and link phases.