Replace source catalog with metadata file

There are two things which this approach should eventally bring :—

1. Availability of various aspects of file metadata for every file
   during the parsing and transformation phases. Right now, only
   metadata for the file currently being processed is available, and
   while loading the catalog is possible in transforms, it’s probably
   not really advisable and hasn’t been extensively tested.

2. Cacheing of file metadata for files which have not changed since the
   last time Make was run. This comes with the usual Make drawback that
   if a file is changed to be older, rather than newer, it won’t be
   recognized as having been changed.

Neither of these things are really implemented at this point, but the
metadata file is created and being used and the old catalog has been
removed. Future commits should refine the behaviour.