Do not require access to dependencies that aren't being documented

It's not always practical to add a dependency on the bzl files that come from third party dependencies. `stardoc` should allow documentation to be generated without introspecting deeply into dependencies.

As an example, I use `MavenInfo` from `rules_jvm_external` in my own rules for building java modules. In order to generate docs using stardoc, I must somehow also include source files from `rules_jvm_external`

The issue names no specific file or test; start by tracing Stardoc's dependency introspection for the MavenInfo example from rules_jvm_external. Done means documentation generation no longer requires adding source bzl files from dependencies that are not being documented.

Written by the indexing model from the issue text.