[dev guide] Built-in modules naming

[Totktonada]

Related dev. issue(s): N/A

Product: Tarantool dev guides
Since: N/A
Audience/target: tarantool developers or external contributors
Root document: somewhere in developer guidelines (neither Lua style guide, nor C style guide fit ideally here, so don't know where exactly it should be added - somewhere in https://www.tarantool.io/en/doc/latest/contributing/guidelines_index/)
SME: @ Totktonada (let's also ask an agreement from other server development teams)

Details

It would be good to formalize rules about built-in modules naming. We started a discussion here and, it seems, agreed on some rules:

1. Use internal.foo for a Lua module that is not exposed into the public API.
   • Existing examples: internal.argparse, internal.trigger, internal.print. (Will be more after this PR.)
2. Use foo.lib for a Lua/C module that is not exposed into the public API and used in the foo module.
   • Existing examples: net.box.lib. (Will be more after this PR.)

Planning checklist

• Pick product label:
  • server for Tarantool core
  • cartridge,
  • tdg, or
  • ecosystem
    for tools and connectors.
• Pick type label:
  • reference,
  • getting started,
  • user_guide, etc.
• Estimate complexity in storypoints in the title
• Add to Documentation board → Backlog

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