cmd/go/internal/modload: don't report path errors when loading retractions

When we load module retractions from the latest version of a module,
it's possible that the latest version has a different module path than
the version we're loading. This can happen when a module is renamed or
a go.mod file is added for the first time.

We should not report an error in this case. Retractions should still
apply to old aliases of a module.

Fixes #41350

Change-Id: If1bc0b6b2b26fc7023e02fc211aa0cd8eb00796e
Reviewed-on: https://go-review.googlesource.com/c/go/+/255961


Trust: Jay Conrod <jayconrod@google.com>
Run-TryBot: Jay Conrod <jayconrod@google.com>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Go Bot <gobot@golang.org>