cmd/compile/internal/types: don't export Nopkg anymore

There's already special code to access it.

Change-Id: I28ca4f44a04262407ee9f1c826ada4e7eba44775
Reviewed-on: https://go-review.googlesource.com/41073
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

src/cmd/compile/internal/gc/reflect.go

@@ -90,7 +90,7 @@ func uncommonSize(t *types.Type) int { // Sizeof(runtime.uncommontype{})
 func makefield(name string, t *types.Type) *types.Field {
 	f := types.NewField()
 	f.Type = t
-	f.Sym = types.Nopkg.Lookup(name)
+	f.Sym = (*types.Pkg)(nil).Lookup(name)
 	return f
 }
 

src/cmd/compile/internal/types/pkg.go

@@ -45,7 +45,7 @@ func NewPkg(path, name string) *Pkg {
 	return p
 }
 
-var Nopkg = &Pkg{
+var nopkg = &Pkg{
 	Syms: make(map[string]*Sym),
 }
 
@@ -58,8 +58,9 @@ var InitSyms []*Sym
 
 // LookupOK looks up name in pkg and reports whether it previously existed.
 func (pkg *Pkg) LookupOK(name string) (s *Sym, existed bool) {
+	// TODO(gri) remove this check in favor of specialized lookup
 	if pkg == nil {
-		pkg = Nopkg
+		pkg = nopkg
 	}
 	if s := pkg.Syms[name]; s != nil {
 		return s, true
@@ -77,8 +78,9 @@ func (pkg *Pkg) LookupOK(name string) (s *Sym, existed bool) {
 }
 
 func (pkg *Pkg) LookupBytes(name []byte) *Sym {
+	// TODO(gri) remove this check in favor of specialized lookup
 	if pkg == nil {
-		pkg = Nopkg
+		pkg = nopkg
 	}
 	if s := pkg.Syms[string(name)]; s != nil {
 		return s