Infer a dep on types-protobuf when running mypy on protobuf-generated code

[benjyw]

Today adding the dep manually allows it to typecheck:

protobuf_sources(
    dependencies = ["3rdparty/python#types-protobuf"],
)

As long as this is set too:

[python-protobuf]
mypy_plugin = true

that is.

We should add this dep automatically.

Note that without types-protobuf you would have to set this in mypy.ini:

[mypy-*.v1.*]
ignore_errors = True

Which would cause mypy to miss some type information from the generated code, which would in turn cause it to miss errors (such as accessing unknown fields) in code that uses the generated code.