Revert "Don't specify a custom module name when generating Podspecs (#916)" (#930)
Revert "Don't specify a custom module name when generating Podspecs (#916)" (#930)

Motivation:

- #916 was really ill thought through and breaks Cocoapods support

Modifications:

- Revert commit fa00728faa93196b513fafa8da6eb79cfe20b117.

Result:

Cocoapods is less broken.
grpc/grpc-swift,grpc/grpc-swift,grpc/grpc-swift,grpc/grpc-swift,grpc/grpc-swift