fix Windows build with latest rules_rust

the extra .lib file that is being output for .dll files was tripping
up the copy; we need to select only the .dll file

pylib/anki/_backend/BUILD.bazel

@@ -2,6 +2,7 @@ load("@rules_python//python:defs.bzl", "py_binary")
 load("@py_deps//:requirements.bzl", "requirement")
 load("@bazel_skylib//rules:copy_file.bzl", "copy_file")
 load("@bazel_skylib//lib:selects.bzl", "selects")
+load(":python_lib.bzl", "copy_python_lib")
 
 py_binary(
     name = "genbackend",
@@ -44,26 +45,9 @@ genrule(
     ],
 )
 
-copy_file(
-    name = "rsbridge_unix",
-    src = "//pylib/rsbridge",
-    out = "rsbridge.so",
-)
-
-copy_file(
-    name = "rsbridge_win",
-    src = "//pylib/rsbridge",
-    out = "rsbridge.pyd",
-)
-
-alias(
+copy_python_lib(
     name = "rsbridge",
-    actual = selects.with_or({
-        (
-            "@rules_rust//rust/platform:x86_64-pc-windows-msvc",
-        ): ":rsbridge_win",
-        "//conditions:default": ":rsbridge_unix",
-    }),
+    src = "//pylib/rsbridge",
 )
 
 _py_srcs = [

pylib/anki/_backend/python_lib.bzl

@@ -0,0 +1,38 @@
+load("@bazel_skylib//lib:paths.bzl", "paths")
+
+def _copy_python_lib_impl(ctx):
+    desired_file = None
+    target_name = ctx.label.name
+    if ctx.attr.is_windows:
+        target_name += ".pyd"
+        for file in ctx.files.src:
+            if file.extension == "dll":
+                desired_file = file
+                break
+    else:
+        target_name += ".so"
+        desired_file = ctx.files.src[0]
+
+    file = ctx.actions.declare_file(target_name)
+    ctx.actions.symlink(output = file, target_file = desired_file)
+
+    return [DefaultInfo(files = depset([file]))]
+
+_copy_python_lib = rule(
+    implementation = _copy_python_lib_impl,
+    attrs = {
+        "src": attr.label(),
+        "is_windows": attr.bool(mandatory = True),
+    },
+)
+
+def copy_python_lib(name, src):
+    "Copy a dynamic library, renaming it to the extension Python expects."
+    _copy_python_lib(
+        name = name,
+        src = src,
+        is_windows = select({
+            "@bazel_tools//src/conditions:host_windows": True,
+            "//conditions:default": False,
+        }),
+    )