Move internal modules to “internal” folder.

.internal/baseFill.js

@@ -0,0 +1,32 @@
+import toInteger from './toInteger.js';
+import toLength from './toLength.js';
+
+/**
+ * The base implementation of `fill` without an iteratee call guard.
+ *
+ * @private
+ * @param {Array} array The array to fill.
+ * @param {*} value The value to fill `array` with.
+ * @param {number} [start=0] The start position.
+ * @param {number} [end=array.length] The end position.
+ * @returns {Array} Returns `array`.
+ */
+function baseFill(array, value, start, end) {
+  const length = array.length;
+
+  start = toInteger(start);
+  if (start < 0) {
+    start = -start > length ? 0 : (length + start);
+  }
+  end = (end === undefined || end > length) ? length : toInteger(end);
+  if (end < 0) {
+    end += length;
+  }
+  end = start > end ? 0 : toLength(end);
+  while (start < end) {
+    array[start++] = value;
+  }
+  return array;
+}
+
+export default baseFill;