Pack is a variant of the Plan 9 ar tool. The original is documented at
It adds a special Go-specific section __.PKGDEF that collects all the Go type information from the files in the archive; that section is used by the compiler when importing the package during compilation.
go tool pack [uvnbailogS][mrxtdpq][P prefix] archive files ...
The new option 'g' causes pack to maintain the __.PKGDEF section as files are added to the archive.
The new option 'S' forces pack to mark the archive as safe.
The new option 'P' causes pack to remove the given prefix from file names in the line number information in object files that are already stored in or added to the archive.