Fix for broken relative imports and missing betterproto_lib_google_protobuf

[BananaLoaf]

The issue affects both relative imports and google.protobuf imports, but is not 100% reproducible.

My working theory is that for service method input types output_package.imports sometimes populates too late to be included in the imports section of a generated file. The list is populated only when template is compiled with method.py_input_message_param which comes way after the import section.

On the contrary, imports for output types are figured out before template compilation in ServiceMethodCompiler:__post__init__ by using self.py_output_message_type

Fix is to add

if "Optional" in self.py_input_message_type:
    self.output_file.typing_imports.add("Optional")

I have only checked it with google.protobuf types, but I suspect that relative imports behave no different and this issue is related to #441

[Dimitrioglo]

Hello guys @danielgtaylor, @Gobot1234,

can you add please this fix in new beta version?
It has huge impact to generated code, i needed to fork your repo to proceed with python interfaces generating for proto files. It's fixes a lot missing return types import errors

[Gobot1234]

I can't fix issues myself with a fast turnaround due to needing a review from a collaborator. If you open a PR with the fix I can take a look