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/build.rs

@@ -0,0 +1,7 @@
+fn main() -> Result<(), Box<dyn std::error::Error>> {
+    tonic_build::configure()
+        .extern_path(".foo.bar.baz.Animal", "crate::Animal")
+        .compile(&["foo.proto"], &["."])?;
+
+    Ok(())
+}