this avoids problems people have run into with
multiple closures in the same package.

when preparing filename, only cut off .go, not .anything.
this fixes a bug tgs ran into with foo.pb.go and foo.go
in the same package.

also turn bad identifier chars from filename into
underscores: a-b.pb.go => a_b_pb

R=ken
OCL=27050
CL=27050