cmd/compile: package-annotate structs when error would be ambiguous

cmd/compile: package-annotate structs when error would be ambiguous

Before emitting a "wanted Foo but got Bar" message for an interface
type match failure, check that Foo and Bar are different.  If they
are not, add package paths to first unexported struct field seen,
because that is the cause (a cause, there could be more than one).

Replicated in go/types.

Added tests to go/types and cmd/compile/internal/types2

Fixes #54258.

Change-Id: Ifc2b2067d62fe2138996972cdf3b6cb7ca0ed456
Reviewed-on: https://go-review.googlesource.com/c/go/+/422914
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: David Chase <drchase@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>