fix(tutorial): import dependencies via meta crate

[indirection42]

Description

Notes & open questions

I'm new to this lib, and I find directly copying the ping example as the tutorial said has some import issues. For example, libp2p_tls is the dependency in the libp2p codebase, but will not be in the application codes only if you specify it. I don't know whether the confusion is a side effect of passing the doctests.

Change checklist

• I have performed a self-review of my own code
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• A changelog entry has been made in the appropriate crates

[thomaseizinger]

Thank you! Those is certainly the recommended way to import these. Perhaps we should extract the tutorial into a different crate to avoid these kind of errors.