build: generate root crate paths correctly (#623)

We shouldn't append `proto_path` (`super` by default) if the path starts
with `crate::`.

Fixes https://github.com/hyperium/tonic/issues/548

tests/root-crate-path/foo.proto

@@ -0,0 +1,10 @@
+syntax = "proto2";
+
+package foo.bar.baz;
+
+message Animal {
+  optional string name = 1;
+}
+service Zoo {
+  rpc process_animal(Animal) returns (Animal) {};
+}