cmd/go: avoid needing to manipulate ImportStack when constructing error

Simplify the printing of PackageErrors by pushing and popping packages
from the import stack when creating the error, rather than when printing
the error. In some cases, we don't have the same amount of information
to recreate the exact error, so we'll print the name of the package
the error is for, even when it's redundant. In the case of import cycle
errors, this change results in the addition of the position information
of the error.

This change supercedes CLs 220718 and 217106. It introduces a simpler
way to format errors.

Fixes #36173

Change-Id: Ie27011eb71f82e165ed4f9567bba6890a3849fc1
Reviewed-on: https://go-review.googlesource.com/c/go/+/224660


Run-TryBot: Michael Matloob <matloob@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>