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

tests/included_service/proto/includee.proto

@@ -0,0 +1,11 @@
+syntax = "proto3";
+
+package includee;
+
+service Included {
+  rpc SomeMethod(SomeRequest) returns (SomeResponse) {}
+}
+
+message SomeRequest {}
+
+message SomeResponse {}

tests/included_service/proto/includer.proto

@@ -0,0 +1,11 @@
+syntax = "proto3";
+
+package includer;
+
+message TopMessage {}
+
+service TopService {
+  rpc TopMethod(TopMessage) returns (TopMessage) {}
+}
+
+import "includee.proto";