SPM support

[3a4oT]

This is POC.

• Depends on https://github.com/pinterest/PINCache/pull/283.

• Use only for SPM https://github.com/SDWebImage/libwebp-Xcode - libwebp wrapper. (submodule pointed to libwebp GitHub mirror)

• The current PINRemoteImage source files layout is hard to adopt for SPM. To allow build with SPM I added the include folder and moved all public headers to it.

• SPM test target is omitted since currently, PINRemoteImageTests contains mixed sources (Objective-C and Swift) which are not supported by SPM. Tests sources should be restructured in the way we could make two separate test targets for Swift sources and Objective-C. As far as we can perform testing using the Xcode project to run tests I think it's fine to leave it as is.

  • Carthage doesn't support Xcode 12 yet. Pinned to Xcode 11.7

[garrettmoon]

@3a4oT Looks good to me! Thank you for putting up this PR!

[3a4oT]

@garrettmoon thanks! Can you please review follow up commits #573?