Rename pathSlashModel to model and remove path flag

[jtoar]

This PR resolves a sub-discussion of #487, and is a continuation of #423. @thedavidprice @cannikin @antonmoiseev

The functionality introduced in #423 remains unchanged--you can still specify a path in the model, but we're no longer calling it pathSlashModel. Just model now, and the functionality will be revealed in a help message:

Output for yarn rw g scaffold --help

~/redwood-app$ yarn rw g scaffold --help
yarn run v1.22.4
$ /redwood-app/node_modules/.bin/rw g scaffold --help
rw g scaffold <model>

Generate Pages, SDL, and Services files based on a given DB schema Model.

Positionals:
  model  Model to scaffold. Optionally may use "path/model" to nest files by
         type at the given path directory (or directories)            [required]

Options:
  --help     Show help                                                 [boolean]
  --version  Show version number                                       [boolean]
  --force                                             [boolean] [default: false]

We're also taking out the path flag here, so now there's only one way to do it.

[thedavidprice]

🚀

yarn rw g scaffold --help looks ✅

yarn rw g --help --> What about adding a hint at the end re: path/model. So:
Generate Pages, SDL, and Services files based on a given DB schema Model. Optionally <path/model> (see 'scaffold --help').

Or something like that. It's getting long. But still needs hint at top-level -- agreed, thoughts?

[jtoar]

Agreed, because I'm not sure if users would look for yarn rw g scaffold --help unless prompted / really stuck, and since yarn rw g --help already gives you most of the information you need to use the commands, it's pretty hidden.

[jtoar]

@thedavidprice new help messages for yarn rw g --help and yarn rw g scaffold --help respectively are (showing only the relevant parts here):

  rw g scaffold <model>    Generate Pages, SDL, and Services files based on a
                           given DB schema Model. Also accepts <path/model> (see
                           'scaffold --help').

rw g scaffold <model>

Generate Pages, SDL, and Services files based on a given DB schema Model. Also
accepts <path/model> (see 'scaffold --help').

Positionals:
  model  Model to scaffold. You can also use <path/model> to nest files by type
         at the given path directory (or directories)--e.g. admin/post[required]

[jtoar]

@thedavidprice Barring anything else, I think this one's ready.

[thedavidprice]

Thanks for the rapid turnaround! This is good on my end. Tested locally and works 💯

@cannikin want to give it a look through before merging?