Summary:
This diff imports and refines the PR made by cntrump on GitHub.
The PR introduces the following:
* Sample apps now use SPM instead of CocoaPods to import IGListKit.
* Adds Mac Catalyst as an example target.
* Adds C++ flags to the CocoaPods specs.
* Fixes a script issue that was discovered when regenerating the symlinks.
The PR originally aimed to remove the need for symlinked references to the IGListKit and IGListDiffKit source files, but in testing, I couldn't get it working. It's possible SPM being too strict [on where the headers can be placed to be discovered](https://forums.swift.org/t/how-do-i-specify-the-headers-directory-for-a-objc-target-in-swift-package-managers-package-swift/58531/3).
Additionally, another issue was that the original PR changed all of the `#import` statements to the the modular `import` statements, which is fine for the sample apps, but ended up breaking compatibility for any apps that had modules disabled.
## Changes in this pull request
Improve SwiftPM support:
Build module `IGListDiffKit` and `IGListKit` as Objective-C++.
module `IGListDiffKit`:
- Source/IGListDiffKit
- module defined in `Source/IGListDiffKit/modulemap/module.modulemap`
- requires `-fmodules` and `-fcxx-modules`
module `IGListKit`:
- depend on `IGListDiffKit`, use `import IGListDiffKit;`
- Source/IGListKit
- module defined in `Source/IGListKit/modulemap/module.modulemap`
- requires `-fmodules` and `-fcxx-modules`
module `IGListSwiftKit`:
- depend on `IGListKit`, use `import IGListKit`
- Source/IGListSwiftKit
Deleted `spm/` and `scripts/generate_spm_sources_layout.sh`, it is unnecessary.
Updated `.podspec`, add `'OTHER_CFLAGS' => '-fmodules'` and `'OTHER_CPLUSPLUSFLAGS' => '-fcxx-modules'`.
Add missing swift files for `IGListSwiftKit` in `IGListKit.xcodeproj`
### How to use
Replace `#import <IGListDiffKit/IGListDiffKit.h>` with `import IGListDiffKit;`, because `IGListDiffKit.h` isn't exist in `modulemap/`, Or create a symbol link by `ln -sf ../IGListDiffKit.h` for support it ?
### Examples
Use SwiftPM for building examples.
### Checklist
- [x] All tests pass. Demo project builds and runs.
- [x] I added tests, an experiment, or detailed why my change isn't tested.
- [ ] I added an entry to the `CHANGELOG.md` for any breaking changes, enhancements, or bug fixes.
- [x] I have reviewed the [contributing guide](https://github.com/Instagram/IGListKit/blob/master/.github/CONTRIBUTING.md)
Pull Request resolved: https://github.com/Instagram/IGListKit/pull/1546
Test Plan: Test PR showing running tests: https://github.com/TimOliver/IGListKit/actions/runs/4339956050/jobs/7578047058
Reviewed By: lorixx
Differential Revision: D33592395
Pulled By: TimOliver
fbshipit-source-id: 8f7b1873f2b1c6a80908bb55b123e31bea13bb0c
