🚸 make dd/Package.hpp export dd/Package_fwd.hpp

[burgholzer]

Description

This PR slighlty improves the usability of the IWYU features of clang-tidy for the DD package. To this end, it marks the dd/Package_fwd.hpp include in the dd/Package.hpp header as an export. This way, header need not include both headers, but can only import the dd/Package.hpp header to also get the content of the foward declaration header.

Checklist:

• The pull request only contains commits that are related to it.
• I have added appropriate tests and documentation.
• I have made sure that all CI jobs on GitHub pass.
• The pull request introduces no new warnings and follows the project's style guidelines.