First part of support for hierarchical module names:

   - the Finder now searches all possible paths when looking for
     a source file or .hi file.  I've removed the caching because
     now we have to search in subdirectories of each path option,
     and it was dubious whether the cache was actually helping.

   - the compilation manager now outputs a warning if it can't find
     the source for a given module, only the .hi file.  Previously
     this caused a cryptic error message when we attempted to call
     getModificationTime on the non-existent source file.