go: implement doc, fmt, fix, list, vet

go: implement doc, fmt, fix, list, vet

This CL is concerned with the basic Package structure
and applies it to the (trivial) implementations of the
doc, fmt, fix, list, and vet commands.

The command as a whole is still very much a work in progress.
In particular, work making the error messages look nice
is deferred to a future CL.

R=golang-dev, adg, dsymonds, r
CC=golang-dev
https://golang.org/cl/5482048