Use prebuilt protoc toolchain

This downloades a prebuilt protoc binary and uses that through regular
toolchain resolution (with the incompatible flag). This increases build
speed and removes the many compiler warnings that pop-up during
compilation of protoc sources. And more importantly works better on
Windows. Where for some reason the protoc compilation fails from an
external module:

    gits/bb-deployments $ bazel build \
        --override_module protobuf=../protobuf \
        @protobuf//src/google/protobuf:timestamp_proto
    ERROR: C:/tmp/eprbnvuo/external/protobuf~/src/google/protobuf/compiler/java/BUILD.bazel:99:11: Compiling src/google/protobuf/compiler/java/context.cc [for tool] failed: undeclared inclusion(s) in rule '@@protobuf~//src/google/protobuf/compiler/java:java':
    this rule is missing dependency declarations for the following files included by 'src/google/protobuf/compiler/java/context.cc':
      'bazel-out/x64_windows-opt-exec-ST-d57f47055a04/bin/external/protobuf~/src/google/protobuf/compiler/java/_virtual_includes/java/google/protobuf/compiler/java/context.h'

Whereas it does work when building from its own checkout:

    ~/gits/protobuf $ bazel build \
        //src/google/protobuf:timestamp_proto
    Target //src/google/protobuf:timestamp_proto up-to-date:
      bazel-bin/src/google/protobuf/timestamp_proto-descriptor-set.proto.bin
    INFO: Build completed successfully, 1 total action

.bazelrc

@@ -37,3 +37,5 @@ build:remote-ubuntu-22-04 --extra_execution_platforms=//tools/remote-toolchains:
 
 # When running bare executors on your own host machine.
 build:remote-local --config=remote-exec
+
+common --incompatible_enable_proto_toolchain_resolution

MODULE.bazel

@@ -22,6 +22,7 @@ bazel_dep(name = "rules_oci", version = "1.7.5")
 bazel_dep(name = "rules_pkg", version = "0.10.1")
 bazel_dep(name = "rules_proto", version = "6.0.0-rc2")
 bazel_dep(name = "toolchains_llvm", version = "1.0.0")
+bazel_dep(name = "toolchains_protoc", version = "0.3.1")
 
 bazel_dep(name = "buildifier_prebuilt", version = "6.4.0", dev_dependency = True)
 

tools/BUILD.bazel

@@ -0,0 +1,27 @@
+load("@rules_proto//proto:defs.bzl", "proto_lang_toolchain")
+
+# proto_lang_toolchain(
+#     name = "protoc_go_toolchain",
+#     command_line = "--go_out=%s",
+#     progress_message = "Generating Go proto_library %{label}",
+#     runtime = "@protobuf-go//jar",
+# )
+
+# toolchain(
+#     name = "protoc_go_toolchain.registration",
+#     toolchain = ":protoc_go_toolchain",
+#     toolchain_type = "@rules_go//go/proto:toolchain_type",
+# )
+
+proto_lang_toolchain(
+    name = "protoc_java_toolchain",
+    command_line = "--java_out=%s",
+    progress_message = "Generating Java proto_library %{label}",
+    runtime = "@protobuf-java//jar",
+)
+
+toolchain(
+    name = "protoc_java_toolchain.registration",
+    toolchain = ":protoc_java_toolchain",
+    toolchain_type = "@rules_java//java/proto:toolchain_type",
+)