Reverts #24337
Original #24289
Fixes #24281

• Applied the fix suggested by Richard to prevent the import order issue if the gRPC module is in a cyclic import and the import path is renamed.
• Also, removed the test case that executes "from grpc import aio", so the unit tests should cover the importing name "grpc.aio" correctly.

Internally, the OSS gRPC module is renamed to "_open_source_grpc", and will be imported in 2 steps: the import hack, the actual import. Checking package name works in this case.

If we use open source mode internally, the OSS gRPC module name stays "grpc", but still it will be imported in 2 steps: a no-op import, the actual import. The tests failed because the "aio" module tried to initiate itself after the "no-op" import and found "grpc" module to be empty.

So, the (brutal-force) solution is to check the file name to ensure when the import hack happened, don't import "aio" module in this file. The import of "aio" module will happen in the magical __init__.py (cl/335754366).