[fidlgen][go] Do not export request/response structs

To bridge between function arguments/returns and encoding/decoding, the
Go bindings generate request/response structs holdings the various
argument/return values. However, developers should not interact which
these, instead using method calls and receiving returns.

With the introduction of the error syntax, the fidlc compiler creates a
name for the struct holding the value, i.e. the first variant of the
result union where the second is the error. The name for this struct is
suffixed by "Response" which used to clash with the response struct
mentioned above.

By keeping the request/response struct private (unexported), we avoid
this clash.

(This does not solve the more general problem of name clashing -- nor
does it try to.)

FIDL-519 #comment

Change-Id: I50767de8d4b3e2d2cfd1b4f7fd4585ead9a455fd