Create `__all__` to explicitly export only the important parts

[MicaelJarniac]

Currently, it generates the following:

# Generated by the protocol buffer compiler.  DO NOT EDIT!
# sources: example.proto
# plugin: python-betterproto
from dataclasses import dataclass

import betterproto


@dataclass
class Greeting(betterproto.Message):
    """Greeting represents a message you can tell a user."""

    message: str = betterproto.string_field(1)

If you try to import from it, you'll get suggestions for importing dataclass, betterproto and Greeting.

Those first two, though, aren't meant to be re-exported there, only Greeting is supposed to be importable.

By generating with an __all__ = ["Greeting"], Python would know that the only thing it's supposed to import is Greeting:

# Generated by the protocol buffer compiler.  DO NOT EDIT!
# sources: example.proto
# plugin: python-betterproto

__all__ = ["Greeting"]

from dataclasses import dataclass

import betterproto


@dataclass
class Greeting(betterproto.Message):
    """Greeting represents a message you can tell a user."""

    message: str = betterproto.string_field(1)

https://docs.python.org/3/tutorial/modules.html#importing-from-a-package https://pep8.org/#module-level-dunder-names