cmd/gc: preserve qualified names of unexported methods in imports.

Fixes #6295.

LGTM=rsc
R=golang-codereviews, rsc
CC=golang-codereviews
https://golang.org/cl/20850043

src/cmd/gc/dcl.c

@@ -941,8 +941,6 @@ interfacefield(Node *n)
 				f->nname = n->left;
 				f->embedded = n->embedded;
 				f->sym = f->nname->sym;
-				if(importpkg && !exportname(f->sym->name))
-					f->sym = pkglookup(f->sym->name, structpkg);
 			}
 
 		} else {

test/fixedbugs/issue6295.dir/p0.go

+// Copyright 2013 The Go Authors.  All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p0
+
+type T0 interface {
+	m0()
+}
+
+type S0 struct{}
+
+func (S0) m0() {}

test/fixedbugs/issue6295.dir/p1.go

+// Copyright 2013 The Go Authors.  All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p1
+
+import "./p0"
+
+type T1 interface {
+	p0.T0
+	m1()
+}
+
+type S1 struct {
+	p0.S0
+}
+
+func (S1) m1() {}
+
+func NewT0() p0.T0 {
+	return S1{}
+}
+
+func NewT1() T1 {
+	return S1{}
+}

test/fixedbugs/issue6295.dir/p2.go

+// Copyright 2013 The Go Authors.  All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package main
+
+import (
+	"./p0"
+	"./p1"
+)
+
+var (
+	_ p0.T0 = p0.S0{}
+	_ p0.T0 = p1.S1{}
+	_ p0.T0 = p1.NewT0()
+	_ p0.T0 = p1.NewT1() // same as p1.S1{}
+)
+
+func main() {}

test/fixedbugs/issue6295.go

+// compiledir
+
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Issue 6295: qualified name of unexported methods
+// is corrupted during import.
+
+package ignored