Rename 'stub' into something else

[evert]

The word stub is used throughout the application. I suspect that in most cases where 'stub' is used, it should actually be something like 'package' or 'zip'.

[Hywan]

We reuse the same terminology as PHP uses.

[evert]

The stub in the phar package is only the php file that automatically gets loaded, which doesn't really apply to for example the zip. The term stub is not as broad as 'the entire package' or 'the thing that creates the package', so I feel a better name could be picked here. I'd suggest just calling it 'Packager', but I think we should actually be able to move this all into the Makefile. This then also solves #283

[Hywan]

Packager is good 😄.