This PR addresses a transitive dependency loss when working with nested
messages.  Previously, we only constructed a descriptor using the direct
dependencies, and this changes the implementation to use the cache of
all encountered descriptors rather than the local list of dependencies.

There's still some opportunities for improvement with regards to
proto3, in particular the wrapper types.  However, I'm deferring that
until we have that sorted out in the backend a bit better.

This also changes the testing slightly.  I've added a comment to the
test as well, but our problem is evaluating the dependencies for
correctness.  Due to the dynamic nature of how descriptors are created
we can use something like proto.Equal.

To address this, we make two comparisons.  In the first, we check the
descriptor directly without its dependencies.  In the second, we
call the normalizer to give us a self-contained descriptor proto, where
we can validate everything is present.  The gap here of course is that
a bug in the normalizer may mask a problem with the conversion, so
consideration is warranted when we revisit this.

Fixes: googleapis#6258