swarm/src/lib: Rework connection exports

[mxinden]

Mostly to unblock @dignifiedquire, i.e. to expose ConnectionLimits. My bad, bug introduced with #2492.

I am surprised that the compiler didn't catch ConnectionLimits not being exposed, given that it is used in the public SwarmBuilder method connection_limits.

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For a follow up pull request:

I am not happy with how we do exports in libp2p-swarm today, mostly as it does not seem to be consistent and thus not intuitive.

• I like the idea of exposing modules like behaviour and protocols_handler as a exports grouping mechanism. Ideally with Adopt naming conventions that don't repeat names #2174, i.e. exposing libp2p_swarm::behaviour::Action and not libp2p_swarm::behaviour::NetworkBehaviourAction.
• I want to prevent both accidental exposure of private types (e.g. Pool should never be exposed) as well as preventing to forget exposing a publicly used type (e.g. ConnectionLimits).

Anyone has opinions / suggestions on the above?

[dignifiedquire]

thanks

[dignifiedquire]

• I like the idea of exposing modules like behaviour and protocols_handler as a exports grouping mechanism. Ideally with Adopt naming conventions that don't repeat names #2174, i.e. exposing libp2p_swarm::behaviour::Action and not libp2p_swarm::behaviour::NetworkBehaviourAction.

💯

[thomaseizinger]

preventing to forget exposing a publicly used type (e.g. ConnectionLimits).

Moving most of the tests into tests/ can help with that. Doctests also run as a separate crate can thus be used to enforce the public API.

[thomaseizinger]

I want to prevent both accidental exposure of private types (e.g. Pool should never be exposed)

Perhaps adding trybuild tests could catch this? It is a bit excessive though 😁