Utilise FilenameUtils.concat for platform independent file path building

[Chris0296]

Description

Throughout the project "/" is used as a delimiter for file paths. This is specific to Linux/Mac OS. This PR utilises Apache Commons IO FilenameUtils.concat function to build platform independent file paths

• Github Issue: https://github.com/cqframework/cqf-tooling/issues/187
• [x] I've read the contribution guidelines
• [x] Code compiles without errors
• [ ] Tests are created / updated
• [ ] Documentation is created / updated

By creating this PR you acknowledge that your contribution will be licensed under Apache 2.0