Fixes the bug tickled by the tests in this commit's parent.

These tests tickle an error serializing emtpy messages as fields of
other messages when using speed-optimized serializers.

This ensures that all pathnames for all .proto files we load get put
in PROTO-IMPL::*ALL-SCHEMAS*, not just those we import.

Don't override file extension for imports; assume the string that was
used to form the pathname is accurate. This makes base-path and
proto-file the same, so fold them both into proto-file.

Drop bogus checks for type "proto".

* FIND-SCHEMA no longer works on a string or keyword. Both of these
  were based on the PATHNAME-NAME of the protobuf, which has a high
  probability of collision. (e.g., common.proto -> "COMMON", :COMMON)

  Instead, look up schemas based on package and name. For instance,
  the protobuf foo.proto, declaring "package FooPackage;", can be
  found with: (FIND-SCHEMA 'FOO-PACKAGE::FOO)

* Pathname-based schema lookup no longer ignores file type.