Problem with grpc

I use `import grpc` in my code. Pipreqs will write both `grpc` and `grpcio` in the requirements.txt.
But `pip install grpc` will fail. It should only write `grpcio`.

Reproduce the issue with a project containing `import grpc` and inspect how pipreqs maps imports to package names. Verify the generated requirements.txt contains `grpcio` rather than the uninstallable `grpc` package.

Written by the indexing model from the issue text.