Discover and load tmt subcommands via entrypoint

[happz]

Instead of enumerating submodules in tmt code, switch the subcommand discovery to entrypoint, and expose subcommands there.

Pull Request Checklist

• [x] implement the feature
• [ ] write the documentation
• [ ] extend the test coverage
• [ ] update the specification
• [ ] adjust plugin docstring
• [ ] modify the json schema
• [ ] mention the version
• [ ] include a release note

[LecrisUT]

If we go with this, we should also discuss the src-layout transition, since it makes less and less sense to not use it.