Problem: Using `vim.pack.del()` to delete active plugin can lead to
a situation when this plugin is reinstalled after restart. Removing
plugin from 'init.lua' is documented, but can be missed.
Solution: Make `del()` only remove non-active plugins by default and
throw an informative error if there is an active plugin.
Add a way to force delete any plugin by adding `opts.force`. This also
makes `del()` signature be the same as other functions, which is nice.
f5707a9c42
