cmd/go: don't assume ImportStack always set on PackageError
When determining DepsErrors for packages, we were trying to sort
errors by the top package on their ImportStack (which would likely be
the package the error was generated for) to get a deterministic
error order.
The problem is that some PackageErrors don't have ImportStacks set on
them. Fall back to sorting the errors by the error text (instead of
making things more complicated by tracking the packages that produced
the errors more closely).
Fixes #59905
Change-Id: Id305e1e70801f8909fd6463383b8eda193559787
Reviewed-on: https://go-review.googlesource.com/c/go/+/501978
Run-TryBot: Michael Matloob <matloob@golang.org> Reviewed-by: David Chase <drchase@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org> Reviewed-by: Bryan Mills <bcmills@google.com>