imports: use LoadFiles everywhere
imports: use LoadFiles everywhere

Since we don't really need type info, and want everything to be fast,
use LoadFiles and do parsing manually. It would be nice if there were a
mode for that.

Change-Id: I33f8a85ffb87a70048c4775058bd0813cf677061
Reviewed-on: https://go-review.googlesource.com/c/155478
Run-TryBot: Heschi Kreinick <e89ab2b172855aa0ad01c40c4d8a3535a61ac68a@google.com>
Reviewed-by: Ian Cottrell <52376736e5e84615ec49f9c1843b8acc178b1d8d@google.com>

golang/tools,golang/tools,golang/tools,golang/tools,golang/tools