Test that dependencies are actually used, not just loaded

[IanButterworth]

I appreciate this is probably hard to do, but just to collect thoughts..

https://github.com/julia-actions/julia-treeshake exists which tests that dependencies are used during tests via code coverage.

It can be fooled by __init__'s.

Might there be a cleverer/more efficient way to do this?

[lgoettgens]

That seems like a great test idea. However, I would only include it into Aqua (at least for the default tests) if it runs in a short time at doesn't need to run the whole testsuite of the package.

If somebody has an idea on how to approach this, please let me know or provide a (partial) PR yourself.