Introduce a pure Swift 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.

In addition to rules_python implementation, this implementation uses a similar mechanism as the C++ implementation for deducing the RUNFILES_DIR and RUNFILES_MANIFEST_FILE location based on argv0.

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

[brentleyjones]

In which cases can it return nil, and should it throw instead?

[cerisier]

It returns nil in 2 scenarios:

1. The runfiles path is relative
2. No manifest entry for this path if using the manifest

I can make it throw instead indeed.