Repackage SDK to be source based

[andrewjl-mux]

### Tasks
- [x] Automated tests for SPM installation
- [x] Automated tests for SPM installation on Mac Catalyst
- [x] Replace MUXSDKStatsTargets with MuxCore binary target in Package.swift
- [x] Convert Swift Package Manager Package.swift to use source-based targets
- [x] Convert Cocoapod podspec to install source files
- [x] Graceful Carthage support for source-based SDK

[gabors]

How is this one coming along?

[andrewjl-mux]

Hey @gabors, no timeline to share yet on when it'll be available. We'll keep this issue posted with any progress. Thank you for checking in!

[andrewjl-mux]

Hey @gabors! Some good news, there's a PR here with what should be most of the required changes to remove MUXSDKStats.xcframework. There's still some more testing to be done so the PR is a draft for now.

[gabors]

Hi @andrewjl-mux How is that 4.0.0 release coming along?