Move internal modules to “internal” folder.

.internal/replaceHolders.js

@@ -0,0 +1,29 @@
+/** Used as the internal argument placeholder. */
+const PLACEHOLDER = '__lodash_placeholder__';
+
+/**
+ * Replaces all `placeholder` elements in `array` with an internal placeholder
+ * and returns an array of their indexes.
+ *
+ * @private
+ * @param {Array} array The array to modify.
+ * @param {*} placeholder The placeholder to replace.
+ * @returns {Array} Returns the new array of placeholder indexes.
+ */
+function replaceHolders(array, placeholder) {
+  let index = -1;
+  const length = array.length;
+  let resIndex = 0;
+  const result = [];
+
+  while (++index < length) {
+    const value = array[index];
+    if (value === placeholder || value === PLACEHOLDER) {
+      array[index] = PLACEHOLDER;
+      result[resIndex++] = index;
+    }
+  }
+  return result;
+}
+
+export default replaceHolders;