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/src/main.rs

@@ -0,0 +1,19 @@
+#[derive(Clone, PartialEq, ::prost::Message)]
+pub struct Animal {
+    #[prost(string, optional, tag = "1")]
+    pub name: ::core::option::Option<::prost::alloc::string::String>,
+}
+
+// pub mod foo;
+
+pub mod foo {
+    pub mod bar {
+        pub mod baz {
+            tonic::include_proto!("foo.bar.baz");
+        }
+    }
+}
+
+fn main() {
+    println!("Hello, world!");
+}