The index file is not generated for the nested client

[JialinHuang803]

When there are nested clients, the index files under the folder of the clients are missing.

For example, in the nestedClient smoke test, there needs to be an index.ts file within the blob folder, which exports all necessary components of this blob sub-client.
https://github.com/Azure/autorest.typescript/tree/main/packages/typespec-test/test/nestedClient/generated/typespec-ts/src/blob