cat-gen: Limit the exported structs and functions

We don't have other modules yet, but let's preemptively limit the structs and
functions exported from this module.

I spotted this when looked at the godoc view of documentation for this package.

Remove unnecessary/leftover functions. Simplify types, for example the type only
defined and used for sorting.

git-svn-id: svn+ssh://svn.code.sf.net/p/gar/code/csw/mgar/gar/v2@23274 d3b55034-1cff-0310-a425-aefe953e1e90
cat-gen: Tweaking log output

Removed one leftover debugging line, and added one meaningful message when
fetching a catalog via REST fails.

git-svn-id: svn+ssh://svn.code.sf.net/p/gar/code/csw/mgar/gar/v2@23275 d3b55034-1cff-0310-a425-aefe953e1e90
cat-get: Remove use of pointers to maps

Map is already a reference type, so it doesn't buy us anything to pass
a pointer to it.