cachi2: generate remote-source.json

[MartinBasti]

Cachi2 doesn't provide remote-source.json but for backward compatibility OSBS must provide it.

STONEBLD-2585

Maintainers will complete the following section

• [ ] Commit messages are descriptive enough
• [ ] Code coverage from testing does not decrease and new code is covered
• [ ] Python type annotations added to new code
• [ ] JSON/YAML configuration changes are updated in the relevant schema
• [ ] Changes to metadata also update the documentation for the metadata
• [ ] Pull request has a link to an osbs-docs PR for user documentation updates
• [ ] New feature can be disabled from a configuration file

[mkosiarc]

LGTM, in ideal world we could use more abstraction by exporting some functionality to individual helper functions so they could be read and understood better (for example, the type matching or version finding). But I am not expecting any change here.

[mkosiarc]

I appreciate the comments, without them I would be completely lost.