Rename tracker mod to core #255

I propose renaming the root module tracker to core.

Mod: https://github.com/torrust/torrust-tracker/tree/develop/src/tracker

$ tree src/core/
src/core/
├── auth.rs
├── databases
│   ├── driver.rs
│   ├── error.rs
│   ├── mod.rs
│   ├── mysql.rs
│   └── sqlite.rs
├── error.rs
├── mod.rs
├── peer.rs
├── services
│   ├── mod.rs
│   ├── statistics
│   │   ├── mod.rs
│   │   └── setup.rs
│   └── torrent.rs
├── statistics.rs
├── torrent.rs
└── tracker.rs

That will allow having these namespaces:

• core::tracker::Tracker
• core::peer::Peer
• Etcetera.

In my opinion, that's better than the:

• tracker::Tracker
• tracker::peer::Peer

The mod contains more things than just the tracker, and that Tracker could even disappear in the future. I think what we have is the domain logic for BitTorrent trackers, regardless of which delivery mechanism you use (the way end-user interact with that domain logic).

I'm writing the documentation for the crate, and it makes sense to give it a name to the "core" tracker logic.

If we move the mod to a package in the future, the name would be torrust-tracker-tracker if we follow current conventions. I think torrust-tracker-core would be a better name.

I'm open to more options. I think core may be too generic.

Other alternatives:

• torrust-tracker-domain
• torrust-bittorrent-tracker

We must consider that we also have another package called shared with logic used by the core, http, udp, ...

cc @da2ce7 @WarmBeer