A few more model fixes to fully support google/protobuf/unittest.proto

A few more model fixes to fully support google/protobuf/unittest.proto
(I love this file, it's got one of everything.)

 - The printer should not print extended fields in any message
   that further extends an extended message.
 - If parsing or importing a .proto file needs a Lisp package that
   does not exist, go ahead and create it.
 - Importing now tracks all the schemas imported by the "parent",
   this so that we can search namespaces thoroughly.
 - It looks like 'define-extend' can accept groups. For a deprecated
   feature, it sure is a pain in the neck.
 - Make 'find-message' and 'find-enum' search all namespaces.
 - Print escaped strings, just for completeness.
 - Annotate a couple of common options with their types.

Passes 'precheckin'. Passes my by-hand tests and is epsilon away from
handling the worst .proto file around.

git-svn-id: http://svn.internal.itasoftware.com/svn/ita/trunk/qres/lisp/quux/protobufs@543912 f8382938-511b-0410-9cdd-bb47b084005c