Move internal modules to “internal” folder.

.internal/assocIndexOf.js

@@ -0,0 +1,21 @@
+import eq from './eq.js';
+
+/**
+ * Gets the index at which the `key` is found in `array` of key-value pairs.
+ *
+ * @private
+ * @param {Array} array The array to inspect.
+ * @param {*} key The key to search for.
+ * @returns {number} Returns the index of the matched value, else `-1`.
+ */
+function assocIndexOf(array, key) {
+  let length = array.length;
+  while (length--) {
+    if (eq(array[length][0], key)) {
+      return length;
+    }
+  }
+  return -1;
+}
+
+export default assocIndexOf;