Prune unused vendored protobuf sources

[Qabbout]

Summary

This PR reduces the amount of vendored upstream protobuf/abseil source that SwiftPM clients need to check out when depending on SwiftProtobuf.

It does this by pruning upstream files that are not used by SwiftProtobuf's package products:

• upstream protoc built-in language generator implementations for C++, C#, Java, Kotlin, Objective-C, PHP, Python, Ruby, and Rust
• upstream protobuf C++ test fixture directories that are not built by this SwiftPM package
• upstream abseil/cctz timezone testdata used by upstream tests, not by SwiftProtobuf package products

It also updates scripts/UpdateProtobufSubtrees.py so future vendored protobuf/abseil refreshes continue to prune the same unused files instead of reintroducing them.

Motivation

Xcode/SwiftPM package resolution can spend a noticeable amount of time checking out SwiftProtobuf when it appears as a transitive dependency in iOS projects. A meaningful part of the package payload comes from vendored upstream protobuf/abseil files.

SwiftProtobuf's protoc executable target uses Sources/protobuf/main.cc, not upstream's full compiler/main.cc. That entry point enables external protoc-* plugins and does not register upstream's built-in language generators. The pruned generator trees therefore are not compiled or exposed by this package target.

The Swift package also has its own test/reference/fuzz/compile-test inputs under Tests/, Protos/, Reference/, FuzzTesting/, and CompileTests/; the removed upstream C++ test fixtures are not part of the SwiftPM build/test graph.

Local impact

Measured locally after this change:

• Sources/protobuf size dropped from about 34M to 28M
• tracked files under Sources/protobuf dropped from 2733 to 1787
• total indexed tracked bytes dropped from about 57.2M to 53.7M

This is intentionally a conservative reduction. It does not change public Swift APIs, package products, target names, or plugin behavior.

Validation

Ran locally on macOS with Apple Swift 6.3.1:

swift package describe
swift build --target protoc
swift test

swift test result:

Executed 958 tests, with 0 failures (0 unexpected)

Notes and limitations

This reduces checkout payload for future commits/releases that include the change. It cannot change the checkout behavior of already-published tags.

A larger improvement would be to split runtime-only consumers from generator/tooling sources so iOS apps that only need the SwiftProtobuf runtime do not need to fetch protoc implementation sources at all. That would be a broader package-structure change; this PR keeps the current package structure and only removes files that are unused by the existing SwiftPM products.

[FranzBusch]

LGTM. @thomasvl @allevato could you give this a look as well?

[thomasvl]

Seems reasonable.

[thomasvl]

@Qabbout - Can you also delete cpp, java, and ruby directories in Reference/upstream/google/protobuf/compiler/. That should fix the CI failures.

[Qabbout]

Done ✅ @thomasvl

[thomasvl]

Shoot. Forgot, since we're removing Sources/protobuf/protobuf/src/google/protobuf/util/python/testdata/test_messages.proto, we also need to remove Reference/upstream/google/protobuf/util/python.

[thomasvl]

@Qabbout I think that will be the last thing needed. You should be able to do a make check to double check what was failing the CI.

[Qabbout]

Thanks for the tip @thomasvl we should be good to go.

[thomasvl]

Thanks again for the patience, merging now.