* DO-PROCESS-IMPORT is now responsible for deciding whether or not to
process an import. It does so by searching for the .proto to be
imported, and upon finding it, checks to see if we alredy have a
schema for that pathname. Additionally, it now returns the pathname.
* PROCESS-IMPORTS and PROCESS-IMPORTS-FROM-FILE now defer the work of
figuring out whether or not an import has been done to
DO-PROCESS-IMPORT. PROCESS-IMPORTS uses the return value of
DO-PROCESS-IMPORT to update the PROTO-IMPORTED-SCHEMAS of the schema
being processed.