Allow lintrunner to run internally (#533)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/533

As titled, I copied `.ci/docker/requirements-lintrunner.txt` to replace the one under the root directory so that we can use it in OSS as well as internal. Then the original `.ci/docker/requirements-lintrunner.txt` becomes a relative symlink pointing to `../../requirements-lintrunner.txt`. That file doesn't exist in internal repo but exists in OSS.

Reviewed By: tarun292, huydhn, dbort

Differential Revision: D49756316

fbshipit-source-id: 3c712602cf7b6daa99a329f0b6a7a735221eab41