cmd/goimports: add -format-only flag
cmd/goimports: add -format-only flag

Fixes golang/go#31745

Change-Id: I127ca4140c9db7df224b1892c43c457ac6edd646
Reviewed-on: https://go-review.googlesource.com/c/tools/+/174326
Run-TryBot: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

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