go/parser: more tolerant parsing of import declarations

go/parser: more tolerant parsing of import declarations

This is a port of CL 427156 from the syntax package's parser
to go/parser.

While at it, remove an unused token.Pos parameter from
parseSpecFunction and dependent declarations.

Also, consolidate the respective test file.

For #54511.

Change-Id: Id6a28eb3d23a46fa5fa1d85d2c4e634b7015513c
Reviewed-on: https://go-review.googlesource.com/c/go/+/427157
Run-TryBot: Robert Griesemer <gri@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>