[dev.regabi] cmd/compile: add ir.PkgName

OPACK was using a whole Node and Name and Param
to hold about three fields. Give it its own implementation.

Passes buildall w/ toolstash -cmp.

Change-Id: I85a28b43d37183b2062d337b0b1b2eea52884e8c
Reviewed-on: https://go-review.googlesource.com/c/go/+/274093
Trust: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

src/cmd/compile/internal/ir/sizeof_test.go

@@ -21,7 +21,7 @@ func TestSizeof(t *testing.T) {
 		_64bit uintptr     // size on 64bit platforms
 	}{
 		{Func{}, 152, 280},
-		{Name{}, 44, 80},
+		{Name{}, 36, 64},
 		{Param{}, 44, 88},
 		{node{}, 88, 152},
 	}