cmd/go: download test dependencies of all named packages

Fixes issue 8181.

LGTM=rsc
R=rsc
CC=golang-codereviews
https://codereview.appspot.com/123870043
flag: mention -h in docs

Fixes issue 8314.

LGTM=r
R=r
CC=golang-codereviews
https://codereview.appspot.com/125820043
encoding/json: document coercion of invalid UTF-8 characters

Fixes issue 8342.

LGTM=iant
R=golang-codereviews, iant
CC=golang-codereviews
https://codereview.appspot.com/122180043