go/types: don't update the underlying type of an imported type

go/types: don't update the underlying type of an imported type

Updating the underlying type of an imported type (even though
is was set to the same type again) leads to a race condition
if the imported package is imported by separate, concurrently
type-checked packages.

Fixes #31749.

Change-Id: Iabb8e8593eb067eb4816c1df81e545ff52d32c6c
Reviewed-on: https://go-review.googlesource.com/c/go/+/201838
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>