cmd/go: detect case-insensitive import path collision

cmd/go: detect case-insensitive import path collision

We already detect this collision when both imports are used
anywhere in a single program. Also detect it when they are in
different targets being processed together.

Fixes #20264.

Change-Id: I5d3c822aae136053fbcb5ed167e1d67f9b847a0f
Reviewed-on: https://go-review.googlesource.com/46424
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>