fix(build): Prevent duplicated client/server generated code (#121)

* 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

tonic-build/src/lib.rs

@@ -262,6 +262,8 @@ impl prost_build::ServiceGenerator for ServiceGenerator {
 
             let code = format!("{}", client_service);
             buf.push_str(&code);
+
+            self.clients = TokenStream::default();
         }
 
         if self.builder.build_server && !self.servers.is_empty() {
@@ -279,6 +281,8 @@ impl prost_build::ServiceGenerator for ServiceGenerator {
 
             let code = format!("{}", server_service);
             buf.push_str(&code);
+
+            self.servers = TokenStream::default();
         }
     }
 }