Support Swift Package Manager

[liamnichols]

The original ios-twitter-image-pipeline repository supported CocoaPods but did not offer support for SPM. To support SPM, we need to do a few things:

1. Restructure target directories to place public headers in an includes directory in order to automatically generate a module map
2. Use XCFramework distributions of WebP (Use xcframework distribution of WebP #1)
3. Correct imports/includes
4. Rework test bundles

In this Pull Request, I do all of the above and also update WebP to 1.2.4 (was 1.2.2).

Note: At the same time, I deleted the original xcodeproj which also housed sample apps. I've moved the code into the Project dir but I have not restored the project itself. The samples don't work anyway because they relied on the Twitter API, so i'm not going to invest the time in getting them working again.