move and refactor verbs into the ament_tools.verbs package

I moved the verbs into `ament_tools.verbs` and I refactored the entry points and imports to reflect the distinction between verbs and commands.

In `catkin_tools` I also prefixed verbs with their corresponding commands. Currently I just moved verbs from the `ament_tools.commands` Python package into the `ament_tools.verbs` Python package without changing the module/package names of the verbs themselves. So, for example, `ament_tools.commands.build_pkg` is now `ament_tools.verbs.build_pkg`. If we follow the `catkin_tools` approach I would change that to `ament_tools.verbs.ament_build_pkg`, but I could also see `ament_tools.verbs.ament.build_pkg` or more verbose `ament_tools.verbs.ament_verbs.build_pkg`/`ament_tools.ament_verbs.build_pkg`.

Regardless of which naming scheme we go with I think it is important to separate commands (first argument on CLI) and verbs (first positional argument) and further more separate verbs for different commands. This allows for multiple commands if we need them in the future and it allows different commands to have verbs with the same name but different implementations.

@dirk-thomas what do you think?