How to use a sub-repo/sub-module in another root module

[mikael-s-persson]

I have basically the following structure:

• "lib_a" is a bazel module that depends on "lib_b" (remote, e.g., git repo)
• "lib_b is a bazel module that depends on "lib_c"
• "lib_c is a bazel module contained within a sub-folder of "lib_b", i.e., in "third_party/lib_c"

In order to build any target in lib_b that refers to @lib_c, I need to "use" that repo at the top level MODULE.bazel in "lib_a". I have tried to follow and adapt the examples that instruct to use a module extension for this purpose, but to no avail.

I have a repro here: mikael-s-persson/bazelbuild_examples@a908775

It has the following files (pretend lib_a and lib_b are two separately hosted packages (git/http)):

./lib_b/deps.bzl
./lib_b/BUILD.bazel
./lib_b/MODULE.bazel

./lib_b/third_party/lib_c/BUILD.bazel
./lib_b/third_party/lib_c/MODULE.bazel

./lib_a/BUILD.bazel
./lib_a/MODULE.bazel

In relevant part. In theory, the lib_b/MODULE.bazel could contain:

local_repository(
    name = "lib_c",
    path = "third_party/lib_c",
)

But that only works to build lib_b targets when it is the root module. Trying to build targets in lib_a that depend on targets from lib_b fails because third_party/lib_c is not a subdirectory of lib_a, obviously.

I tried a bunch of different things (lost count), and some of which I noted in the code I linked to above. Nothing seems to work so far. The best I could manage was to create a relative_repository rule that is like local_repository but also takes a parent repository label (e.g., @lib_b) and assumes the local path to be relative to where that repo is located from the workspace root, but that fails at the lib_a level because it can't find that directory, although I can definitely see that it exists where bazel it doesn't (in the build cache folder).

Long story short, how do I make this work?
How should lib_b use lib_c such that lib_b is also buildable as a dependency, not just as the root module?
How can lib_a make @lib_c accessible too?