[release-branch.go1.15] cmd/go/internal/load: always set IsImportCycle when in a cycle

When hitting an import cycle in reusePackage, and there is already
an error set, make sure IsImportCycle is set so that we don't
end up stuck in a loop.

Updates #25830
Fixes #47347

Change-Id: Iba966aea4a637dfc34ee22782a477209ac48c9bd
Reviewed-on: https://go-review.googlesource.com/c/go/+/301289


Trust: Roland Shoemaker <roland@golang.org>
Run-TryBot: Roland Shoemaker <roland@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Jay Conrod <jayconrod@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
(cherry picked from commit cdd08e61)
Reviewed-on: https://go-review.googlesource.com/c/go/+/336669
Trust: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>