Don't clean as part of sync

[BlueDrink9]

Currently, swapping one plugin for another cannot be done temporarily without having to reinstall plugins after launch. This is because a sync operation cleans out unused plugin files. Is it possible with the pack mechanism to please avoid this behavior?

Reason:

I have separate plugins added used depending on whether I am using nvim or vim, or whether I want a faster light vim for config files or a more bloated vim for IDE-style coding. I want to share the same plugin files, to save space on disk and download time.

Currently, if I load one set, sync, then load the other set the new set will no longer have half of its plugins, because they were not part of set 1 and were deleted during the sync.