`pack` command gives wrong help message and doesn't support packing a directory

### What needs to get done

Provide the correct parameters in the help message and provide a way to pack a directory.

### Why it needs to get done

- `*craft pack --help` says to use a part name, even though this raises an error:
```bash
~> testcraft pack --help
Usage:
testcraft pack [options]
```
- There is no straightforward way for an application to override the pack command to pack a directory because the command requires loading a project file.

Source: https://github.com/canonical/snapcraft/issues/4769

Start by locating the pack command's help definition and the code that loads the project file. Check how command arguments are parsed, then verify that the help shows the correct parameters and that a directory can be packed without requiring a project file.

Written by the indexing model from the issue text.