Change handling of LocalSource in queries

1. Change `LocalSource` to extend `DataFlow::Node`, thus removed from
   the definiton of `Source`
2. Add a private class `AddLocalSource` which extends `Source`. This
   allows us to currently preserve the inclusion of local sources, while
   making it easier to remove it in the future.

Author: egregius313

csharp/ql/lib/semmle/code/csharp/security/dataflow/CodeInjectionQuery.qll

@@ -66,7 +66,9 @@ deprecated class RemoteSource extends DataFlow::Node instanceof RemoteFlowSource
  *
  * A source of local user input.
  */
-deprecated class LocalSource extends Source instanceof LocalFlowSource { }
+deprecated class LocalSource extends DataFlow::Node instanceof LocalFlowSource { }
+
+private class AddLocalSource extends Source instanceof LocalFlowSource { }
 
 /** A source supported by the current threat model. */
 class ThreatModelSource extends Source instanceof ThreatModelFlowSource { }

csharp/ql/lib/semmle/code/csharp/security/dataflow/ResourceInjectionQuery.qll

@@ -65,7 +65,9 @@ deprecated class RemoteSource extends DataFlow::Node instanceof RemoteFlowSource
  *
  * A source of local user input.
  */
-deprecated class LocalSource extends Source instanceof LocalFlowSource { }
+deprecated class LocalSource extends DataFlow::Node instanceof LocalFlowSource { }
+
+private class AddLocalSource extends Source instanceof LocalFlowSource { }
 
 /** A source supported by the current threat model. */
 class ThreatModelSource extends Source instanceof ThreatModelFlowSource { }

csharp/ql/lib/semmle/code/csharp/security/dataflow/SqlInjectionQuery.qll

@@ -76,7 +76,9 @@ deprecated class RemoteSource extends DataFlow::Node instanceof RemoteFlowSource
  *
  * A source of local user input.
  */
-deprecated class LocalSource extends Source instanceof LocalFlowSource { }
+deprecated class LocalSource extends DataFlow::Node instanceof LocalFlowSource { }
+
+private class AddLocalSource extends Source instanceof LocalFlowSource { }
 
 /** A source supported by the current threat model. */
 class ThreatModelSource extends Source instanceof ThreatModelFlowSource { }