Currently, in order to populate the Edit > Color Scheme menu, MacVim searches
for color schemes only in the paths defined in `runtimepath`. Since MacVim is
compiled with `+packages`, it makes sense to also look for color schemes under
`packpath`. This commit addresses this deficiency.
Note that color schemes may be found below `pack/*/opt` or
`pack/*/start` (see `:h pack-add`): `:colorscheme` searches both
locations.
c2ff332dca
