go.tools/go/types: blank package name should not be accepted

[griesemer]

This code is not rejected by go/types:

package _

(Package names must not be blank).

[gopherbot]

Comment 1:

CL https://golang.org/cl/91640043 mentions this issue.

[griesemer]

Comment 2:

This issue was closed by revision golang/tools@8df7a77.

Status changed to Fixed.