Append newly installed pack to file_search_path(pack, ...)

[mgondan]

To reproduce the problem:
$ rm -rf .local/share/swi-prolog/pack/
$ swipl
$ pack_install(date_time)
$ pack_install(environ) -> installed in the wrong folder ("tap")
$ file_search_path(pack, Dir). -> illustrates the problem

The patch appends the new directory to the path list.

[JanWielemaker]

The real problem is that installing a pack should not change the pack search path. That search path searches for pack directories, i.e., the directories that hold pack (directories). Pushed d1defee to address this.