mass-scan: Create library to collect a whole project version from Software Heritage

SWH does not expose a file system and is mostly file-based, we are PURL-based. Given a bunch of file-by-file scans we need to be able to stich them back in a package afterwards.  Or we need to address SWH as if it were a "Virtual Codebase".

For this we should create a library to collect a whole project version from Software Heritage that would then present this as a virtual filesystem.