Don't require absolute paths for cache hits when compiling Rust

[Xanewok]

The more broad issue is tracked upstream at mozilla/sccache#35.

We're mostly interested in supporting only Rust for now and it seems that groundwork has been laid already via --remap-path-prefix. This issue should probably be addressed before we can deploy cachepot as the cache hit rate will be user-specific and thus fairly low initially.

[drahnr]

Related #37

[Xanewok]

FWIW basic support has been implemented via #116. Should we leave this open and track other missed cases and/or unless we can safely enable this feature by default? @drahnr @montekki

[drahnr]

I think we should split this into separate, mores specific issues:

• Assure dwarf debug info stays correct in dist builds #124
• Assure C/C++ depedencies are path independent #123
• ...

[drahnr]

Closing this, reference this issue as needed for follow-up issues :)