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/Cargo.toml

@@ -0,0 +1,14 @@
+[package]
+name = "root-crate-path"
+version = "0.1.0"
+authors = ["Lucio Franco <[email protected]>"]
+edition = "2018"
+publish = false
+license = "MIT"
+
+[dependencies]
+tonic = { path = "../../tonic" }
+prost = "0.7"
+
+[build_dependencies]
+tonic-build = { path = "../../tonic-build" }