implement PathSearcher

to facilitate module reloading

src/utils.rs

@@ -0,0 +1,6 @@
+use std::path::PathBuf;
+
+/// Return the value of `$CARGO_MANIFEST_DIR` at runtime.
+pub(crate) fn runtime_root() -> PathBuf {
+    PathBuf::new().join(std::env::var("CARGO_MANIFEST_DIR").unwrap())
+}