cmd/go: add ImportComment to Package struct

It seems reasonable that people might want to look up the
ImportComment with "go list".

LGTM=r
R=golang-codereviews, r
CC=golang-codereviews
https://codereview.appspot.com/143600043
doc: use "keyed" instead of "tagged" in Go 1 compatibility doc

LGTM=bradfitz, r
R=r, bradfitz
CC=golang-codereviews
https://codereview.appspot.com/156730043