go/printer, gofmt: use blank to separate import rename from import path

go/printer, gofmt: use blank to separate import rename from import path

Note that declarations.golden is not using spaces for alignment (so
that the alignment tabs are visible) which is why this change affects
the test cases significantly. gofmt uses spaces for alignment (by default)
and only tabs for indentation.

gofmt -w src misc (no changes)

Fixes #1673.

R=iant
CC=golang-dev
https://golang.org/cl/4388044