vim_configurable: Add packPath option to vimrcConfig (#22776)

* vim_configurable: Add packages option to vimrcConfig

Version 8 of vim adds the concept of "vim packages": directories which
contain one or more vim plugins, in either "start" or "opt"
subdirectories. Those in "start" are to be loaded automatically, while
those in "opt" can be loaded manually. Vim detects any packages located
in one of its "packpaths".

The packages option takes a set of sets describing one or more vim
packages, and adds the derivation containing these packages to the
packpath.

* fix documentation.