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