Fix integration of local packages with Objective C targets

[fortmarek]

Resolves #5955

Short description 📝

A project with a local Swift package that contains objc code would fail to build. This is because we'd:

• Generate a module map into the Derived directory (a different directory is used for remote packages)
• And then we would always delete it. Since the DeleteDerivedDirectoryProjectMapper is always run after the SPM graph is loaded, the modulemap would never be preserved

For now, I make sure the mapper does not delete the modulemap. This is not an optimal solution but one that works for now. As we make Package.swift first-class citizens, we will also move away from preloading the whole SPM graph and running operations on it before the rest of the graph – instead, the SPM Package.swift manifests will be treated the same way as any other manifest.

How to test the changes locally 🧐

app_with_spm_dependencies should build (I also tried the example in the attached issue)

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