go/types: don't parse imported packages multiple times.
go/types: don't parse imported packages multiple times.

R=dave, golang-dev, rsc
CC=golang-dev
https://codereview.appspot.com/7068044

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang