cmd/compile: implement package-level aliases (no export yet)

Requires -newparser=1.

For #17487.
For #16339.

Change-Id: I156fb0c0f8a97e8c72dbbfbd7fe821efee12b957
Reviewed-on: https://go-review.googlesource.com/31597
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

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

@@ -943,6 +943,8 @@ func mkpackage(pkgname string) {
 					s.Def.Name.Pack.Used = true
 				}
 
+				// TODO(gri) This will also affect exported aliases.
+				// Need to fix this.
 				s.Def = nil
 				continue
 			}