[modules] Introduce new VariableLocation for module imports/exports.

src/ast/prettyprinter.cc

Index: src/ast/prettyprinter.cc
diff --git a/src/ast/prettyprinter.cc b/src/ast/prettyprinter.cc
index 40a60fc601f871bf82503b0b44f356e83ae0db3f..50d716484e6267aabd6851d0860e3dec5a75d547 100644
--- a/src/ast/prettyprinter.cc
+++ b/src/ast/prettyprinter.cc
@@ -1061,6 +1061,9 @@ void AstPrinter::VisitVariableProxy(VariableProxy* node) {
       case VariableLocation::LOOKUP:
         SNPrintF(buf + pos, " lookup");
         break;
+      case VariableLocation::MODULE:
+        SNPrintF(buf + pos, " module");
+        break;
     }
     PrintLiteralWithModeIndented(buf.start(), var, node->name());
   }