Automatic sorting of load order

[MythicManiac]

Add support for automatically sorting mod load order, e.g. by calling gpm autosort.

This issue is not specifying the sorting algorithm that should be used, just the interface that's exposed.

Lightweight specs:

• The autosort should be triggered by a command e.g. gpm autosort (does not have to be this)
• The autosort should ensure the package.lock is up to date (everything defined in dependencies is accounted for)
• The autosort should sort the package.lock with a sorting algorithm (to be defined in another issue)

We could also support more sorting algorithms than one if there's a need to, although I doubt this will be the case.