fix(bzlmod): allow both root module and our module to call cuda.local_toolchain

[cloudhan]

Superserde #239

In #263 (comment) a new error popup as expected.

It is caused by the unittest secretly setup our module (rules_cuda, current root module as of invocation of bazel command) as an external module under testing, then the local_cuda is ignore in this case. We shall allow call to module_extenion in both root module and our module.

This in principle also fixes #238.

[jsharpe]

If a user doesn't have an install at the default path does this cause a failure if they also override it in their own module file? Should we be loading the extension with dev_dependency = True to avoid this if so?

[cloudhan]

No, this does not trigger analysis error anymore thanks to #209. The generated local_cuda repo is as follows

root@c578fa5b9b4e ~/rules_cuda (cloudhan/our-module-local-cuda)# tree bazel-rules_cuda/external/_main\~toolchain\~local_cuda/
bazel-rules_cuda/external/_main~toolchain~local_cuda/
|-- BUILD -> /root/rules_cuda/cuda/runtime/BUILD.local_cuda_disabled
|-- REPO.bazel
|-- WORKSPACE
|-- defs.bzl
`-- toolchain
    |-- BUILD
    |-- clang
    |   `-- BUILD
    `-- disabled
        `-- BUILD -> /root/rules_cuda/cuda/templates/BUILD.local_toolchain_disabled

3 directories, 7 files
root@c578fa5b9b4e ~/rules_cuda (cloudhan/our-module-local-cuda)# cat bazel-rules_cuda/external/_main\~toolchain\~local_cuda/BUILD
load("@bazel_skylib//rules:common_settings.bzl", "bool_setting")

package(
    default_visibility = ["//visibility:public"],
)

bool_setting(
    name = "valid_toolchain_found",
    build_setting_default = False,
)