Move internal modules to “internal” folder.

.internal/baseSet.js

@@ -0,0 +1,48 @@
+import assignValue from './.internal/assignValue.js';
+import castPath from './.internal/castPath.js';
+import isIndex from './.internal/isIndex.js';
+import isObject from './isObject.js';
+import toKey from './.internal/toKey.js';
+
+/**
+ * The base implementation of `set`.
+ *
+ * @private
+ * @param {Object} object The object to modify.
+ * @param {Array|string} path The path of the property to set.
+ * @param {*} value The value to set.
+ * @param {Function} [customizer] The function to customize path creation.
+ * @returns {Object} Returns `object`.
+ */
+function baseSet(object, path, value, customizer) {
+  if (!isObject(object)) {
+    return object;
+  }
+  path = castPath(path, object);
+
+  const length = path.length;
+  const lastIndex = length - 1;
+
+  let index = -1;
+  let nested = object;
+
+  while (nested != null && ++index < length) {
+    const key = toKey(path[index]);
+    let newValue = value;
+
+    if (index != lastIndex) {
+      const objValue = nested[key];
+      newValue = customizer ? customizer(objValue, key, nested) : undefined;
+      if (newValue === undefined) {
+        newValue = isObject(objValue)
+          ? objValue
+          : (isIndex(path[index + 1]) ? [] : {});
+      }
+    }
+    assignValue(nested, key, newValue);
+    nested = nested[key];
+  }
+  return object;
+}
+
+export default baseSet;