cmake: allow overriding the package file name

[VDanielEdwards]

Subject

Description

Instructions for review / testing

Developer checklist (address before review)

• [ ] Changelog.md updated
• [ ] Prepared update for depending repositories
• [ ] Documentation updated (public API changes only)
• [ ] API docstrings updated (public API changes only)
• [ ] Rebase → commit history clean
• [x] Squash and merge → proper PR title

[VDanielEdwards]

Postponed, due to lack of neccessity.