There are at least 2 for Plan 9, one simply called 'contrib' which installs from iso images in sources/contrib, and an entirely different one for 9front called 'pkg'. Years ago, I opposed development of pkg because 9front was pretty-much my main OS and I was burned out on the problems dependencies bring. The big problems for me were continuously maintaining each package and the fact that package managers encourage thoughtless dependency use which compounds the need for maintenance. The latter problem is described very well in Our Software Dependency Problem:
All of the problems diminish if you have time and energy to spare. A relatively static underlying OS also helps a lot. Plan 9 and Inferno are quite static, 9front not so much. Some packages on pkg eventually broke because 9front is continuously updated but package maintainers come and go.
I'm not aware of any package manager for Inferno, but I don't think it would be hard to port either contrib or pkg. They're both in rc shell script. rc is closer to mash than sh but not quite 100% compatible if I remember right. Neither contrib nor pkg were included in their respective OSs. If I remember right, contrib is in sources/contrib/fgb and depends on Plan 9's /rc/bin/replica/* which is also somewhere in the Sources tree. Sources is served over 9p; I don't remember how to mount it from Inferno. I don't know where to get pkg from.