[bzlmod] Cross-module artifact resolution (single-version policy)

Can some documentation be included on the right way to use the module extension when rules_jvm_external is used by multiple dependencies or by a dependency and the root module?

Example dependency graphs, where "->" indicates "depends on":

1. B -> A
2. C -> B -> A
3. C -> A and C -> B

Supposing each of A, B, and C above use the module extension to include Maven artifacts. What is the right way to configure the modules such that they share Maven artifact resolution (i.e. if both A and B include some Maven artifact X we only get one copy/version of X)?

In WORKSPACE mode, we handle this by having each project expose its list of artifacts and only having the root WORKSPACE call `maven_install`.

Start from the module extension behavior described in the issue and trace the three dependency graphs: B -> A, C -> B -> A, and C -> A plus C -> B. Compare this with the existing WORKSPACE approach using a root maven_install, then document the configuration that shares Maven artifact resolution across modules and include examples for each graph.

Written by the indexing model from the issue text.