go/internal/gcimporter: backport changes from x/tools/go/gcimporter15

See https://go-review.googlesource.com/32581.

This makes x/tools/go/gcimporter15/bimport.go a close copy again
and the importer more robust.

Change-Id: If96ad6acd611878b7dfa6a13d005d847ece82ab6
Reviewed-on: https://go-review.googlesource.com/32647
TryBot-Result: Gobot Gobot <gobot@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
Reviewed-by: Alan Donovan <adonovan@google.com>

src/go/internal/gcimporter/bimport.go

@@ -278,11 +278,13 @@ func (p *importer) obj(tag int) {
 		p.declare(types.NewFunc(pos, pkg, name, sig))
 
 	case aliasTag:
-		aliasPos := p.pos()
-		aliasName := p.string()
-		pkg, name := p.qualifiedName()
-		obj := pkg.Scope().Lookup(name)
-		p.declare(types.NewAlias(aliasPos, p.pkgList[0], aliasName, obj))
+		pos := p.pos()
+		name := p.string()
+		var orig types.Object
+		if pkg, name := p.qualifiedName(); pkg != nil {
+			orig = pkg.Scope().Lookup(name)
+		}
+		p.declare(types.NewAlias(pos, p.pkgList[0], name, orig))
 
 	default:
 		errorf("unexpected object tag %d", tag)
@@ -343,7 +345,9 @@ var (
 
 func (p *importer) qualifiedName() (pkg *types.Package, name string) {
 	name = p.string()
-	pkg = p.pkg()
+	if name != "" {
+		pkg = p.pkg()
+	}
 	return
 }