Handle name conflicts in cmg

[raphael]

For each client method the cmg tool generates a Add<method name> and a Set<method name> method on the mock to set the implementation. There is currently no protection against existing client method with identical names which causes the tool to generate duplicate method names.

One approach might be for the tool to use scopes when generating method names so it can keep track of already generated names and append suffixes as necessary, a simpler version of https://github.com/goadesign/goa/blob/v3/codegen/scope.go.