go/parser: parse incomplete selection "fmt." as a blank selection "fmt._"

go/parser: parse incomplete selection "fmt." as a blank selection "fmt._"

Formerly it would return a BadExpr.

This prevents partial syntax from being discarded, and makes the error
recovery logic more consistent with other places where an identifier
was expected but not found.

+ test

Change-Id: I223c0c0589e7ceb7207ae951b8f71b9275a1eb73
Reviewed-on: https://go-review.googlesource.com/10269
Reviewed-by: Robert Griesemer <gri@golang.org>