Move internal modules to “internal” folder.

.internal/baseSlice.js

@@ -0,0 +1,31 @@
+/**
+ * The base implementation of `slice` without an iteratee call guard.
+ *
+ * @private
+ * @param {Array} array The array to slice.
+ * @param {number} [start=0] The start position.
+ * @param {number} [end=array.length] The end position.
+ * @returns {Array} Returns the slice of `array`.
+ */
+function baseSlice(array, start, end) {
+  let index = -1,
+      length = array.length;
+
+  if (start < 0) {
+    start = -start > length ? 0 : (length + start);
+  }
+  end = end > length ? length : end;
+  if (end < 0) {
+    end += length;
+  }
+  length = start > end ? 0 : ((end - start) >>> 0);
+  start >>>= 0;
+
+  const result = Array(length);
+  while (++index < length) {
+    result[index] = array[index + start];
+  }
+  return result;
+}
+
+export default baseSlice;