Manually load altis/core in Composer plugin

[rmccue]

Fixes #501.

Using Composer 2.1.9 locally, confirmed this fixes the command for me.

[roborourke]

Should we add it as a peer dep suggestion or requirement with version * too?

[kovshenin]

Should we add it as a peer dep suggestion or requirement with version * too?

If this package is not intended to be used on its own, I don't think we need to add the requirement here, or to any other Altis module that uses Altis\get_config(). We should also remove the "Installation without Altis" section from the readme if that's the case, then for #497 we can suggest trying to composer require --dev altis/core with no guarantee that it will continue working in the future.

[rmccue]

If it's not onerous for us, we can add the dep, but otherwise I'd say whatever is least painful for us.