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

[patrickomatic]

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.