Avoid quadratic work for deep dependency trees

[varungandhi-src]

The default implementation of cross-repository navigation duplicates work
across nodes of a dependency tree, so that package information for forward
declarations can be identified correctly. We should provide a way to avoid
this quadratic work, with minimal setup steps.

The different possibilities are outlined in the Limitations section in the docs: https://github.com/sourcegraph/scip-clang/blob/main/docs/CrossRepo.md#limitations

[varungandhi-src]

Loosely related:

• Better way to handle repositories with lots of submodules #360