fix(build): Don't replace extern_paths (#261)

* don't replace extern_path'd paths

tonic-build will replace all paths except for google well known types
with one rooted at the super module. For paths that have already been
replaced with a fully qualified path via the extern_path config option,
(e.g. "::uuid::Uuid"), this results in an invalid path
(e.g. "super::::uuid::Uuid"), and a build failure. These paths should
also be excluded when prefixing relative modules with super.

* add doc in tonic-build clarifying extern_path

extern_path expects fully qualified proto and rust paths

* add test cast case for extern path fix

add a test that extern path does indeed result in service types using
the specified type from an external crate. we test this by creating a
service type that has a proto from a different crate, and asserting that
it does indeed impl a trait from that crate

* add license/publish to extern_path test crates

tests/extern_path/proto/my_application/service.proto

@@ -0,0 +1,25 @@
+syntax = "proto3";
+
+package my_application;
+
+option go_package = "my_applicationpb";
+option java_multiple_files = true;
+option java_outer_classname = "ServiceProto";
+option java_package = "com.my_application";
+
+
+import "uuid.proto";
+
+message MyMessage {
+    uuid.Uuid message_id = 1;
+    string some_payload = 2;
+}
+
+service MyService {
+    rpc GetUuid(MyMessage) returns (uuid.Uuid){
+
+    }
+    rpc GetMyMessage(uuid.Uuid) returns (MyMessage){
+
+    }
+}

tests/extern_path/proto/uuid/uuid.proto

@@ -0,0 +1,13 @@
+syntax = "proto3";
+
+package uuid;
+
+option go_package = "uuidpb";
+option java_multiple_files = true;
+option java_outer_classname = "UuidProto";
+option java_package = "com.uuid";
+
+
+message Uuid {
+    string uuid_str = 1;
+}