* fix(build): Prevent duplicated client/server generated code
The tonic-build process collects up RPC services as they are provided by
prost, before writing them out as part of the finalization step for a
given protocol buffer package.
In the case of imported protocol buffer packages, there may be RPC
services included by import in addition to those in the top-level
package. Therefore it is necessary to make sure each set of
client/server services gathered by tonic-build is cleared after the
finalization process for a given protocol buffer package, otherwise they
will be incorrectly aggregated as the generation process proceeds
through the subsequent packages.
* Test case for duplicated client/server generated code
A simple test case that will fail to build without a fix to prevent
RPC services being duplicated into inappropriate modules (that related
to particular protocol buffer packages).
* Additional test case for included_service
Introduces an additional case that captures making sure services defined
before including a package with additional services doesn't
incidentially clear such precursor services from the including package.
* Fix unnecessary newline to keep `cargo fmt` happy
