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/my_application/build.rs

@@ -0,0 +1,11 @@
+fn main() -> Result<(), std::io::Error> {
+    tonic_build::configure()
+        .build_server(false)
+        .build_client(true)
+        .extern_path(".uuid", "::uuid")
+        .compile(
+            &["service.proto", "uuid.proto"],
+            &["../proto/my_application", "../proto/uuid"],
+        )?;
+    Ok(())
+}