Change the default executable name to match the basename of the source
file containing the Main module (or the module specified by -main-is),
if there is one.  On Windows, the .exe extension is added.

As requested on the ghc-users list, and as implemented by Tomasz
Zielonka <tomasz.zielonka at gmail.com>, with modifications by me.

I changed the type of the mainModIs field of DynFlags from Maybe
String to Module, which removed some duplicate code.