Use `raise-user-error' for `raco pkg ...' errors, so that stack
traces don't print out for external errors. Reformat error messages
generally to match current conventions. Use logging for debugging
output.
For now, "METADATA.rktd" is still recognized as a fallback.
Also, rewrite package source type and name inference,
make ".zip" the default format for `raco pkg create',
and many doc edits.
