Fix loading of cached files from a different `ModuleLoader.loader_root`

This is a minor bug but exposes a gap in the file caching scheme.  To reproduce:

* `cd playground/module_loader && make`
* `cargo test`

and the `module_loading` integration test will fail because it's confused where the object code is.  The cached file will have it as `file1.csvpo` and the compiler will think it's `playground/module_loader/file1.csvpo`.  