feat: Add support for manually specifiying gems for cross-repo.

[varungandhi-src]

Motivation

This would help customers specify cross-repo mappings in an easy way
even when our inference fails.

Test plan

• Add unit test for JSON parsing
• Manually test paths with some files. Main thing to check here is both relative and absolute paths in the JSON are accepted.
• Add test for the stdlib version being used. We aren't using specific versions, we can add that later.
• Add support for multi-gem tests (this is a TODO in the test runner).
• Add multi-gem tests where the JSON file is passed from outside. We need to add tests for both relative and absolute paths.
  • Relative path - snapshot test
  • Absolute path - unit test

[varungandhi-src]

Interesting; if I put a print statement inside enterFile, that ends up using GitHub links for RBI paths like

https://github.com/sorbet/sorbet/tree/master/rbi/stdlib/yaml.rbi

[varungandhi-src]

OK, so this does seem to be working OK for basic file lists, but the fallback to the --gem-metadata value is leading to misassignment of package name/version for stdlib stuff like Kernel#puts etc. We should fix that.

[varungandhi-src]

Erm, why is this 'C' and not 'C1' and why are parameter types missing?