Introduce a runfiles library

[cerisier]

First shot at implementing #890

Main implementation

I followed guidance from @fmeum and based this implementation on the runfiles library of rules_python where applicable since this was pointed as the reference implementation.

The non applicable parts, especially the runtime computation of the runfiles directory has been inspired by the implementation in rules_rust.

TODO

• Implement runfiles directory detection from argv0
• Make a proper PR description describing the API, the heuristics behind directory discovery and path lookup.
• Discuss the proper way to solve the current repository issue and whether to keep the BazelRunfilesConstants file generation.
• Fix Linux compat