Move internal modules to “internal” folder.

.internal/baseSortedIndex.js

@@ -0,0 +1,41 @@
+import baseSortedIndexBy from './.internal/baseSortedIndexBy.js';
+import identity from './identity.js';
+import isSymbol from './isSymbol.js';
+
+/** Used as references for the maximum length and index of an array. */
+const MAX_ARRAY_LENGTH = 4294967295;
+const HALF_MAX_ARRAY_LENGTH = MAX_ARRAY_LENGTH >>> 1;
+
+/**
+ * The base implementation of `sortedIndex` and `sortedLastIndex` which
+ * performs a binary search of `array` to determine the index at which `value`
+ * should be inserted into `array` in order to maintain its sort order.
+ *
+ * @private
+ * @param {Array} array The sorted array to inspect.
+ * @param {*} value The value to evaluate.
+ * @param {boolean} [retHighest] Specify returning the highest qualified index.
+ * @returns {number} Returns the index at which `value` should be inserted
+ *  into `array`.
+ */
+function baseSortedIndex(array, value, retHighest) {
+  let low = 0;
+  let high = array == null ? low : array.length;
+
+  if (typeof value == 'number' && value === value && high <= HALF_MAX_ARRAY_LENGTH) {
+    while (low < high) {
+      const mid = (low + high) >>> 1;
+      const computed = array[mid];
+      if (computed !== null && !isSymbol(computed) &&
+          (retHighest ? (computed <= value) : (computed < value))) {
+        low = mid + 1;
+      } else {
+        high = mid;
+      }
+    }
+    return high;
+  }
+  return baseSortedIndexBy(array, value, identity, retHighest);
+}
+
+export default baseSortedIndex;