fix(scripts): skip downleveling of packages in private folder

[trivikr]

Issue

Fixes: #4912

Description

Skips downleveling of packages in private folder

Testing

ToDo: Verified that yarn build:types:downlevel succeeded with the repo setup prior to fix in #4911

$ yarn build:all && yarn build:types:downlevel
...
Done in 114.59s.

Verified that downlevel types were created for clients.

$ ls clients/client-s3/dist-types/ts3.4
commands  index.d.ts  pagination  runtimeConfig.browser.d.ts  runtimeConfig.native.d.ts  S3Client.d.ts  waiters
endpoint  models      protocols   runtimeConfig.d.ts          runtimeConfig.shared.d.ts  S3.d.ts

$ ls private/aws-echo-service/dist-types/ts3.4
ls: cannot access private/aws-echo-service/dist-types/ts3.4: No such file or directory

---

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[github-actions]

This thread has been automatically locked since there has not been any recent activity after it was closed. Please open a new issue for related bugs and link to relevant comments in this thread.