rpc: Use definitions directly instead of via generated code

Until now “autotools/build-rpc” would read the definition of all RPCs
and write them to a new file, “lib/_generated_rpc.py” with some
modifications. With this patch the generated code loads the definitions
and, at module load time, gets references to the actual definitions.
Further patches will even remove the calls to the post-processing
functions from the generated code.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>