cc @rsc @bcmills @suzmue @ianthehat

go/packages.Load allows users to supply contains: as a pattern argument. All other patterns supplied to Load are accepted by go list and it would be helpful if contains: could be also. That would allow go/packages.Load to always make a single go list call.

go list contains: would list the packages containing the given file. This is an operation that tools commonly need, and have approximated in the past by determining the directory of the package.