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

…dows.

Fixes aspect-build#667

This takes the approach [rules_proto_grpc does](https://github.com/rules-proto-grpc/rules_proto_grpc/blob/9e61a4c3dfc78de163febe7530bd04dabfbcca16/modules/core/internal/protoc.bzl#L79-L83) to detect if we are building on Windows via the `host_path_separator`. protoc will fail to call into the plugins if they are expressed to protoc as forward-slash paths.

ts/private/ts_proto_library.bzl

@@ -6,6 +6,16 @@ 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."""
+    # This is a hack to check if we are on Windows, where we assume Windows
+    # if the PATH environment variable is split by ";", and POSIX is split by
+    # ":", exclusively. This is the same approach rules_proto_grpc takes to
+    # normalize Windows paths of plugins handed to protoc.
+    if ctx.configuration.host_path_separator == ";":
+        return path.replace("/", "\\")
+    return path
+
 # buildifier: disable=function-docstring-header
 def _protoc_action(ctx, proto_info, outputs):
     """Create an action like
@@ -29,19 +39,19 @@ 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.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.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 = "=")