Fix compiling libraries with umbrella headers that have a dash

[fortmarek]

Resolves #6037

Short description 📝

The swift-markdown-ui library has two headers in the public headers path:

• cfmark-gfm.h
• and cfmark_gfm.h

When comparing with the generated module map by SPM, it turns out we should only use the header that has the sanitized version (dashes replaced with underscores)

How to test the changes locally 🧐

The app_with_spm_dependencies fixture that now has the swift-markdown-ui as a dependency should build.

Contributor checklist ✅

• The code has been linted using run mise run lint:fix
• The change is tested via unit testing or acceptance testing, or both
• The title of the PR is formulated in a way that is usable as a changelog entry
• In case the PR introduces changes that affect users, the documentation has been updated

Reviewer checklist ✅

• The code architecture and patterns are consistent with the rest of the codebase
• Reviewer has checked that, if needed, the PR includes the label changelog:added, changelog:fixed, or changelog:changed, and the title is usable as a changelog entry