gen-go_cli: rpc methods are not namespaces in filename output

Right now you can't generate a binary that targets more than one service with the same rpc name: `list.go: Tried to write the same file twice.`.

Start in the gen-go_cli filename-output path and reproduce the case with two services sharing an RPC name. Check how list.go is selected and verify that generation no longer reports a duplicate write, with distinct filenames for the methods.

Written by the indexing model from the issue text.