Put the fake Fuchsia SDK in a module extension

This allows users to override the fake SDK with a real one using
https://bazel.build/rules/lib/globals/module#override_repo.

Without this change, it is impossible for a project that depends on
googletest as a bazel_dep to build tests using the "real" Fuchsia SDK,
because any references to @fuchsia_sdk unconditionally resolve to the
"fake" Fuchsia SDK. With this change, you have the real Fuchsia SDK
declared in your MODULE.bazel, you can add the following lines to coerce
googletest to use the real Fuchsia SDK as well:

    fake_fuchsia_sdk_extension = use_extension("@com_google_googletest//:fake_fuchsia_sdk.bzl", "fuchsia_sdk")
    override_repo(fake_fuchsia_sdk_extension, "fuchsia_sdk")

MODULE.bazel

@@ -39,29 +39,37 @@ module(
 # Only direct dependencies need to be listed below.
 # Please keep the versions in sync with the versions in the WORKSPACE file.
 
-bazel_dep(name = "abseil-cpp",
-          version = "20240116.2")
+bazel_dep(
+    name = "abseil-cpp",
+    version = "20240116.2",
+)
+bazel_dep(
+    name = "platforms",
+    version = "0.0.10",
+)
+bazel_dep(
+    name = "re2",
+    version = "2024-07-02",
+)
 
-bazel_dep(name = "platforms",
-          version = "0.0.10")
-
-bazel_dep(name = "re2",
-          version = "2024-07-02")
-
-bazel_dep(name = "rules_python",
-          version = "0.34.0",
-          dev_dependency = True)
+bazel_dep(
+    name = "rules_python",
+    version = "0.34.0",
+    dev_dependency = True,
+)
 
 # https://rules-python.readthedocs.io/en/stable/toolchains.html#library-modules-with-dev-only-python-usage
 python = use_extension(
     "@rules_python//python/extensions:python.bzl",
     "python",
-    dev_dependency = True
+    dev_dependency = True,
+)
+python.toolchain(
+    ignore_root_user_error = True,
+    is_default = True,
+    python_version = "3.12",
 )
 
-python.toolchain(python_version = "3.12",
-                 is_default = True,
-                 ignore_root_user_error = True)
-
-fake_fuchsia_sdk = use_repo_rule("//:fake_fuchsia_sdk.bzl", "fake_fuchsia_sdk")
-fake_fuchsia_sdk(name = "fuchsia_sdk")
+fuchsia_sdk = use_extension("//:fake_fuchsia_sdk.bzl", "fuchsia_sdk")
+fuchsia_sdk.create_fake()
+use_repo(fuchsia_sdk, "fuchsia_sdk")

fake_fuchsia_sdk.bzl

@@ -31,3 +31,21 @@ fake_fuchsia_sdk = repository_rule(
         ),
     },
 )
+
+_create_fake = tag_class()
+
+def _fuchsia_sdk_impl(module_ctx):
+    create_fake_sdk = False
+    for mod in module_ctx.modules:
+        for _ in mod.tags.create_fake:
+            create_fake_sdk = True
+
+    if create_fake_sdk:
+        fake_fuchsia_sdk(name = "fuchsia_sdk")
+
+    return module_ctx.extension_metadata(reproducible = True)
+
+fuchsia_sdk = module_extension(
+    implementation = _fuchsia_sdk_impl,
+    tag_classes = {"create_fake": _create_fake},
+)