go.tools/go/types: add Package.SetImports
go.tools/go/types: add Package.SetImports

R=adonovan, neelance
CC=golang-dev
https://codereview.appspot.com/32500043

lattera/go.tools,chenjianlong/go.tools,chenjianlong/go.tools,karmakaze/go.tools,karmakaze/go.tools,chenjianlong/go.tools,karmakaze/go.tools,lattera/go.tools,karmakaze/go.tools,lattera/go.tools,karmakaze/go.tools