pkg/group: allow lookup from specified file
pkg/group: allow lookup from specified file

Allow looking up gids from a specified file. This makes gid validation
easier.

yifan-gu/rkt,lucasallan/rkt,cab105/rkt,lucasallan/rkt,squeed/rkt,akshaykarle/rkt,dgonyeo/rkt,achanda/rkt,jakon89/rkt,cab105/rkt,jonboulle/rkt,nhlfr/rkt,il9ue/rkt,sstabellini/rkt,jonboulle/rkt,markllama/rkt,coreos/rkt,matthaias/rkt,s-urbaniak/rkt,alban/rkt,euank/rkt,shobhit85/rkt,yifan-gu/rkt,markllama/rkt,rkt/rkt,tmrts/rkt,squeed/rkt,lucasallan/rkt,jakon89/rkt,akshaykarle/rkt,Wilybald/rkt,lucasallan/rkt,alban/rkt,jonboulle/rkt,euank/rkt,sgotti/rkt,yifan-gu/rocket,monstermunchkin/rkt,kinvolk/rkt,euank/rkt,Wilybald/rkt,coreos/rkt,monstermunchkin/rkt,markllama/rkt,Shashwatsh/rkt,achanda/rkt,mike1729/rkt,jakon89/rkt,jjlakis/rkt,shobhit85/rkt,akshaykarle/rkt,intelsdi-x/rkt,intelsdi-x/rkt,yifan-gu/rocket,ybubnov/rkt,monstermunchkin/rkt,rkt/rkt,cab105/rkt,sgotti/rkt,alban/rkt,s-urbaniak/rkt,rkt/rkt,coreos/rkt,mike1729/rkt,intelsdi-x/rkt,shobhit85/rkt,il9ue/rkt,dgonyeo/rkt,sstabellini/rkt,yifan-gu/rkt,achanda/rkt,jjlakis/rkt,matthaias/rkt,tmrts/rkt,ybubnov/rkt,Wilybald/rkt,joshix/rkt,kinvolk/rkt,nhlfr/rkt,sstabellini/rkt,kinvolk/rkt,joshix/rkt,joshix/rkt,il9ue/rkt,tmrts/rkt,rkt/rkt,Shashwatsh/rkt,joshix/rkt,Wilybald/rkt,mike1729/rkt,matthaias/rkt,jjlakis/rkt,kinvolk/rkt,s-urbaniak/rkt,Shashwatsh/rkt,dgonyeo/rkt,squeed/rkt,yifan-gu/rocket,sgotti/rkt,ybubnov/rkt,nhlfr/rkt