fix: respect package namespaces when resolving message descriptors

[ermul]

Overview

This change fixes a couple of bugs exposed by @stan-is-hate in #59 where package namespaces are not taken into account when looking up protobuf messages.

Testing

Ran integrated_tests/imported_message in a loop ~20 times without a failure locally.

[YOU54F]

Hey thanks @ermul

Can you rebase against main which should resolve the CI failures, expect the Pact Verify job, which is expected to fail on contributor PR's from external repos. (Probably want to add a condition to skip that job in future)

[YOU54F]

Also thank you for the fix and @stan-is-hate for the investigation 🙌🏾