Version-indepdent urls for source (non-pod) files

[rwstauner]

When we added the /pod controller we expanded the namespace and created version-independent urls (based on distribution instead of release).
We specified (in #176) that we might do the same for non-pod urls for consistency.
#1094 and #1097 have specifically requested that (version-independent links to, for example, META files).

[haarg]

This endpoint exists now, but isn't yet used for anything except the search.cpan.org redirector.