Fix command_to_module_name not converting snakecased names properly

Currently, snake cased project names did not get converted properly into modules from `Mix.Utils.command_to_module_name`, i.e.:

```elixir
Mix.Utils.command_to_module_name "my_app.task"
=> "My_app.Task"
```

This PR uses `camelize` to properly convert the commands to module namespaces.
It doesn't seem we use `first_to_upper` anymore. Plus `module_name_to_command` uses `first_to_lower` which we should probably fix and remove too. :)