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