go/parser: parse import specs the same way as the syntax parser

go/parser: parse import specs the same way as the syntax parser

This results in better error recovery and allows us to use the
same tests for go/types and types2.

For #54511.

Change-Id: Ic11a9dafb8c62e0cb952b3924d55a28b438241c6
Reviewed-on: https://go-review.googlesource.com/c/go/+/427154
Run-TryBot: Robert Griesemer <gri@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@google.com>