Replace the pack command, a C program, with a clean reimplementation in Go.
It does not need to reproduce the full feature set and it is no longer used by
the build chain, but has a role in looking inside archives created by the build
chain directly.

Since it's not in C, it is no longer build by dist, so remove it from cmd/dist and
make it a "tool" in cmd/go terminology.

Fixes #2705

R=rsc, dave, minux.ma, josharian
CC=golang-codereviews
https://golang.org/cl/52310044