Add external file scope

Author: owen-mc

go/ql/lib/semmle/go/dataflow/internal/DataFlowNodes.qll

@@ -23,7 +23,12 @@ module Private {
     or
     not n instanceof FlowSummaryNode and
     not exists(n.getEnclosingCallable()) and
-    result.asFileScope() = n.getFile()
+    (
+      result.asFileScope() = n.getFile()
+      or
+      not exists(n.getFile()) and
+      result.isExternalFileScope()
+    )
     or
     result.asSummarizedCallable() = n.(FlowSummaryNode).getSummarizedCallable()
   }

go/ql/lib/semmle/go/dataflow/internal/DataFlowPrivate.qll

@@ -256,6 +256,7 @@ class DataFlowLocation = Location;
 private newtype TDataFlowCallable =
   TCallable(Callable c) or
   TFileScope(File f) or
+  TExternalFileScope() or
   TSummarizedCallable(FlowSummary::SummarizedCallable c)
 
 class DataFlowCallable extends TDataFlowCallable {
@@ -269,6 +270,11 @@ class DataFlowCallable extends TDataFlowCallable {
    */
   File asFileScope() { this = TFileScope(result) }
 
+  /**
+   * Holds if this `DataFlowCallable` is an external file scope.
+   */
+  predicate isExternalFileScope() { this = TExternalFileScope() }
+
   /**
    * Gets the `SummarizedCallable` corresponding to this `DataFlowCallable`, if any.
    */