Fixes a regression in update process added in #115
Fixes a regression in update process added in #115

As #115 migrated list functions, after updating Oh My Fish code
the omf.list_installed_packages gets removed, but is still being
used by the update code.

This commit fixes it by providing a wrapper function.

bsott/oh-my-fish,MaicoTimmerman/oh-my-fish,lfiolhais/oh-my-fish,MaicoTimmerman/oh-my-fish,oranja/oh-my-fish,eugenesvk/oh-my-fish,oranja/oh-my-fish,jeremiejig/oh-my-fish,scorphus/oh-my-fish,wfscheper/oh-my-fish,wfscheper/oh-my-fish,lfiolhais/oh-my-fish,bsott/oh-my-fish,wangyung/oh-my-fish,hfs/oh-my-fish,hfs/oh-my-fish,nwykes/oh-my-fish,Pyppe/oh-my-fish,wangyung/oh-my-fish,scorphus/oh-my-fish,oh-my-fish/oh-my-fish,oh-my-fish/oh-my-fish,jeremiejig/oh-my-fish,nwykes/oh-my-fish,bobthecow/oh-my-fish,sheldon/oh-my-fish,eugenesvk/oh-my-fish