JS: Add some helpers

javascript/ql/lib/semmle/javascript/Files.qll

@@ -75,6 +75,19 @@ class Folder extends Container, Impl::Folder {
       )
   }
 
+  /**
+   * Gets an implementation file and/or a typings file from this folder that has the given `stem`.
+   * This could be a single `.ts` file or a pair of `.js` and `.d.ts` files.
+   */
+  File getJavaScriptFileOrTypings(string stem) {
+    exists(File jsFile | jsFile = this.getJavaScriptFile(stem) |
+      result = jsFile
+      or
+      not jsFile.getFileType().isTypeScript() and
+      result = this.getFile(stem + ".d.ts")
+    )
+  }
+
   /** Gets a subfolder contained in this folder. */
   Folder getASubFolder() { result = this.getAChildContainer() }
 }

javascript/ql/lib/semmle/javascript/NPM.qll

@@ -12,12 +12,21 @@ class PackageJson extends JsonObject {
     this.isTopLevel()
   }
 
+  /** Gets the folder containing this `package.json` file. */
+  Folder getFolder() { result = this.getJsonFile().getParentContainer() }
+
+  /**
+   * Gets the name of this package as it appears in the `name` field.
+   */
+  pragma[nomagic]
+  string getDeclaredPackageName() { result = this.getPropStringValue("name") }
+
   /**
    * Gets the name of this package.
    * If the package is located under the package `pkg1` and its relative path is `foo/bar`, then the resulting package name will be `pkg1/foo/bar`.
    */
   string getPackageName() {
-    result = this.getPropStringValue("name")
+    result = this.getDeclaredPackageName()
     or
     exists(
       PackageJson parentPkg, Container currentDir, Container parentDir, string parentPkgName,