fix: convert forward slashes to backslashes for protoc plugins on Win…

…dows.

Fixes aspect-build#667

This takes the approach used [elsewhere in rules_ts](https://github.com/aspect-build/rules_ts/blob/da284adac1fcb6bd9da81ce5d4b3e29660ecface/ts/private/ts_project.bzl#L80) to detect if we are building on Windows. protoc will fail to call into the plugins if they are expressed to protoc as forward-slash paths.

ts/private/ts_proto_library.bzl

@@ -1,11 +1,19 @@
 "Private implementation details for ts_proto_library"
 
 load("@aspect_bazel_lib//lib:copy_to_bin.bzl", "COPY_FILE_TO_BIN_TOOLCHAINS")
+load("@aspect_bazel_lib//lib:platform_utils.bzl", "platform_utils")
 load("@rules_proto//proto:defs.bzl", "ProtoInfo", "proto_common")
 load("@rules_proto//proto:proto_common.bzl", proto_toolchains = "toolchains")
 
 _PROTO_TOOLCHAIN_TYPE = "@rules_proto//proto:toolchain_type"
 
+def _windows_path_normalize(ctx, path):
+    """Changes forward slashs to backslashs for Windows paths."""
+    host_is_windows = platform_utils.host_platform_is_windows()
+    if host_is_windows
+        return path.replace("/", "\\")
+    return path
+
 # buildifier: disable=function-docstring-header
 def _protoc_action(ctx, proto_info, outputs):
     """Create an action like
@@ -29,19 +37,19 @@ def _protoc_action(ctx, proto_info, outputs):
         fail("protoc_gen_options.target must be 'js+dts'")
 
     args = ctx.actions.args()
-    args.add_joined(["--plugin", "protoc-gen-es", ctx.executable.protoc_gen_es.path], join_with = "=")
+    args.add_joined(["--plugin", "protoc-gen-es", _windows_path_normalize(ctx, ctx.executable.protoc_gen_es.path)], join_with = "=")
     for (key, value) in options.items():
         args.add_joined(["--es_opt", key, value], join_with = "=")
     args.add_joined(["--es_out", ctx.bin_dir.path], join_with = "=")
 
     if ctx.attr.gen_connect_es:
-        args.add_joined(["--plugin", "protoc-gen-connect-es", ctx.executable.protoc_gen_connect_es.path], join_with = "=")
+        args.add_joined(["--plugin", "protoc-gen-connect-es", _windows_path_normalize(ctx, ctx.executable.protoc_gen_connect_es.path)], join_with = "=")
         for (key, value) in options.items():
             args.add_joined(["--connect-es_opt", key, value], join_with = "=")
         args.add_joined(["--connect-es_out", ctx.bin_dir.path], join_with = "=")
 
     if ctx.attr.gen_connect_query:
-        args.add_joined(["--plugin", "protoc-gen-connect-query", ctx.executable.protoc_gen_connect_query.path], join_with = "=")
+        args.add_joined(["--plugin", "protoc-gen-connect-query", _windows_path_normalize(ctx, ctx.executable.protoc_gen_connect_query.path)], join_with = "=")
         for (key, value) in options.items():
             args.add_joined(["--connect-query_opt", key, value], join_with = "=")
         args.add_joined(["--connect-query_out", ctx.bin_dir.path], join_with = "=")