Import most third party libraries as Swift packages

[crazytonyli]

Description

This PR replaces most third-party libraries with their Swift Package. That'll bring us much closer to fully get rid of CocoaPods.

From now on, we should add new dependencies as Swift Package. Once new Swift packages are added, you can add them to targets that depend on them.

I imagine, eventually, we can take advantage of the "Modules" local package, where we can add shared code and dependencies. Which will save us from manually click buttons in Xcode and changes the pbxproj files.

Details

The Swift Packages are added manually by using the "Package Dependencies" tab in Xcode, which is a pretty terrible experience comparing to editing a plain text Podfile.

WPKit and WPAuthenticator

Using Swift Package is possible because we now have WPKit and WPAuthenticator inside the repository (see #23366). WPKit, which WPAuthenticator depends, doesn't have full SPM support yet. WPAuthentication of couase can't support SPM because it depends on WPKit. WPAuthenticator depends on third party libraries (like SVProgressHUD) that the apps also depend on. The fact there are common dependencies prevent us from using SPM for the app, because we can't import those common dependencies as Swift Packages and CocoaPods pods.

This PR adds two new targets in the Xcode project, for WPKit and WPAuthenticator. They are plain and simple Xcode targets, which are not integrated with CocoaPods. That means we have to do some "wiring" ourselves.

These two new targets of course still have their dependencies. Their dependencies are added to their target configuration manually. They come from two places: Swift Packages in the "Package Dependencies" of the Xcode project (like SVProgressHUD) and CocoaPods pods (like WordPressUI).

Declaring dependencies ultimately lead to two things: compiled library binary are linked to the end product and library resources are copied to the end product bundle. For Swift Packages, Xcode takes care both for us. However, we have to do some manual configuration to achieve the same with CocoaPods dependencies.

First, we need to set compiler flags to link the dependencies, which is done in the xcconfig files in the config directory. Second, we need to copy dependencies' resources bundles, which is done in the scripts in Scripts/BuildPhases/<WordPressKit|Authenticator> directories. Those scripts are run as build phases in their test targets.

Other targets

WPKit and WPAuthenticator are the only slightly complicated part. The rest of Swift package integration is pretty plain and simple, because we don't need to think about CocoaPods.

Tests

Regression tests with the installable builds in this PR, especially on features that uses libraries which use external resources (storyboards, images, etc). For example, WPAuthenticator uses storyboard to present its UI.

Next steps

We should create a Test Flight build to do a regression tests.

There are still some libraries (mostly our own ones) in the Podfile. We should look into replacing them with Swift Package one by one.

We can start with adding SPM support to MediaEditor, since it depends on CropViewController which is also a dependency of Kanvas. It should be pretty straightforward to support SPM in MediaEdtiro, since it looks like a pure Swift library.

WordPressShared already supports SPM. However, due to the way it integrates SwiftLint, Xcode fails to add it as Swift Package. See wordpress-mobile/WordPress-iOS-Shared#358.

WordPressUI also supports SPM, but due to this Xcode bug, Xcode fails to compile the project when it's added as SPM. The same issue may also occur with WordPressShared Swift Package.

I think we should try to get WordPressShared and WordPressUI integrated with Swift Packages, which will simplify WPKit and WPAuthenticator setup a lot: The "CopyResources" and "EmbedFrameworks" build phase scripts can be deleted.

[dangermattic]

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📲 You can test the changes from this Pull Request in WordPress Alpha by scanning the QR code below to install the corresponding build.

	App Name	WordPress Alpha
	Configuration	Release-Alpha
	Build Number	pr23378-68cf81b
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