move resolveMainPath into a separate helper predicate

Author: erik-krogh

javascript/ql/lib/semmle/javascript/NodeModuleResolutionImpl.qll

@@ -90,21 +90,18 @@ bindingset[name]
 private string getStem(string name) { result = name.regexpCapture("(.+?)(?:\\.([^.]+))?", 1) }
 
 /**
- * Gets the main module described by `pkg` with the given `priority`.
+ * Gets a file that a main module from `pkg` exported as `mainPath` with the given `priority`.
+ * `mainPath` is "." if it's the main module of the package.
  */
-File resolveMainModule(PackageJson pkg, int priority) {
-  exists(PathExpr main, int subPriority, string mainPath |
-    main = MainModulePath::of(pkg, mainPath) and
-    if mainPath = "." then subPriority = priority else priority = subPriority + 1000
-  |
-    result = main.resolve() and subPriority = 0
+private File resolveMainPath(PackageJson pkg, string mainPath, int priority) {
+  exists(PathExpr main | main = MainModulePath::of(pkg, mainPath) |
+    result = main.resolve() and priority = 0
     or
-    result = tryExtensions(main.resolve(), "index", subPriority)
+    result = tryExtensions(main.resolve(), "index", priority)
     or
     not main.resolve() instanceof File and
     exists(int n | n = main.getNumComponent() |
-      result =
-        tryExtensions(main.resolveUpTo(n - 1), getStem(main.getComponent(n - 1)), subPriority)
+      result = tryExtensions(main.resolveUpTo(n - 1), getStem(main.getComponent(n - 1)), priority)
     )
     or
     // assuming the files get moved from one dir to another during compilation:
@@ -114,9 +111,19 @@ File resolveMainModule(PackageJson pkg, int priority) {
       // is in one folder below the package.json, and has the right basename
       result =
         tryExtensions(subFolder, getStem(main.getComponent(main.getNumComponent() - 1)),
-          subPriority - 999)
+          priority - 999) // very high priority, to make sure everything else is tried first
     )
   )
+}
+
+/**
+ * Gets the main module described by `pkg` with the given `priority`.
+ */
+File resolveMainModule(PackageJson pkg, int priority) {
+  exists(int subPriority, string mainPath |
+    result = resolveMainPath(pkg, mainPath, subPriority) and
+    if mainPath = "." then subPriority = priority else priority = subPriority + 1000
+  )
   or
   exists(Folder folder, Folder child |
     child = folder or