JS: Factor out overridden part of PathExpr.getSearchRoot

javascript/ql/src/semmle/javascript/NodeModuleResolutionImpl.qll

@@ -112,7 +112,7 @@ class MainModulePath extends PathExpr, @json_string {
 
   override string getValue() { result = this.(JSONString).getValue() }
 
-  override Folder getSearchRoot(int priority) {
+  override Folder getAdditionalSearchRoot(int priority) {
     priority = 0 and
     result = pkg.getFile().getParentContainer()
   }

javascript/ql/src/semmle/javascript/Paths.qll

@@ -194,8 +194,18 @@ abstract class PathExpr extends Locatable {
   Folder getSearchRoot(int priority) {
     // We default to the enclosing module's search root, though this may be overridden.
     getEnclosingModule().searchRoot(this, result, priority)
+    or
+    result = getAdditionalSearchRoot(priority)
   }
 
+  /**
+   * INTERNAL. Use `getSearchRoot` instead.
+   *
+   * Can be overridden by subclasses of `PathExpr` to provide additional search roots
+   * without overriding `getSearchRoot`.
+   */
+  Folder getAdditionalSearchRoot(int priority) { none() }
+
   /** Gets the `i`th component of this path. */
   string getComponent(int i) { result = getValue().(PathString).getComponent(i) }
 
@@ -276,8 +286,8 @@ private class ConcatPath extends PathExpr {
     )
   }
 
-  override Folder getSearchRoot(int priority) {
-    result = this.(AddExpr).getAnOperand().(PathExpr).getSearchRoot(priority)
+  override Folder getAdditionalSearchRoot(int priority) {
+    result = this.(AddExpr).getAnOperand().(PathExpr).getAdditionalSearchRoot(priority)
   }
 }