Better way to handle repositories with lots of submodules

[varungandhi-src]

Given the state of dependency management in C++, some repositories rely on git submodules for consuming 3rd party dependencies. For example, the boost monorepo has 161 submodules. The Espressif IoT Development Framework has 22 submodules.

At the moment, scip-clang doesn't have any awareness about git submodules. In principle, we could make use of the submodule structure to generate indexes for every package at once (assuming all code is indexed by a top-level compile_commands.json), and either provide a JSON file or a shell script which can be further used to invoke src CLI to upload indexes for all the relevant packages.

(Note: This issue is currently just an "idea", waiting to collect some more input on if this would be useful and what exactly would be needed.)