Tao He authored Jun 18, 2018 and Andrey Mokhov committed Jun 18, 2018

* For executables, we should read the `main-is` field from the cabal file.

Previously, we simply treat file name for `Main` module as `Main.hs` to
build executable. That doesn't work for the `timeout` program. This patch
fixes the problem.

* Add comments about the processing of `main-is` field from .cabal file.