fix: register descriptor dependencies before AddSerializedFile (#150)

[drewfustin]

Description

Fixes #150.

When google_protobuf_descriptors is enabled, generated modules call default_google_proto_descriptor_pool.AddSerializedFile(...) at import time. That fails if a proto imports another file whose descriptor is not already in the pool, like google/protobuf/descriptor.proto for custom options:

TypeError: Couldn't build proto file into descriptor pool: Depends on file 'google/protobuf/descriptor.proto', but it has not been loaded

With this fix, the compiler:

1. Emits relative imports of each foreign dependency package immediately before AddSerializedFile so those modules register first (the workaround suggested in Using options results in descriptor pool error in generated code at import time. #150).
2. Topologically sorts same-package input files by FileDescriptorProto.dependency before emitting registrations.

Note:
Package-level import cycles are not handled. The proto file graph is expected to be a DAG. File-granular registration would be a follow-up if maintainers want it.

Minimal repro from #150 (now covered by tests)

syntax = "proto3";
package descriptor_options;
import "google/protobuf/descriptor.proto";
extend google.protobuf.MessageOptions {
  string example_option = 50000;
}
message MyMessage {
  option (example_option) = "bar";
  string baz = 1;
}

Compiled with --python_betterproto2_opt=google_protobuf_descriptors now imports without error.

Type of change

• Bug fix
• New feature
• Breaking change
• Documentation / tooling

Checklist

• New behaviour is covered by tests
• Documentation updated if needed

Made with Cursor (Claude Fable 5 high + Composer 2.5)
and me ;)