This is to allow monitoring programs for changes. The combination of the
location where the program was found, and all the other locations where
the program was looked for gives the full set of files to monitor for
changes in that program.

The Program programFindLocation method is extended to return the
locations looked at but where the prog was not found. The default
implementation of programFindLocation, findProgramOnSearchPath, is
extended to return those locations.

Other places have to change, mostly just the type. In a couple places in
GHC & GHCJS where there is additional searching done, the not-found
locations have to be collected and returned.