** Previously, "hidden-modules" had nothing to do with what gets
   built, and we had to compute "exposed-modules" with "modules `setdiff`
   hidden modules".  At Ross Paterson's suggestion, I added
   Exposed-Modules.

** Since the executable sections also used the "modules" field, I
   changed it to use "executable-modules", and got rid of the
   ambiguous "modules" field.  This should hopefully be more clear to
   everyone.