chore: fix Carthage submodule structure and SPM xcodeproj structure

[thedavidharris]

This address multiple issues that were affecting the xcodeproj, depending how it was used:

• There's a .gitmodules file present but the submodule doesn't exist, as there's not .git/modules entry. This adds the submodule, properly linked with Carthage's install process.
• The xcodeproj was looking for the "rxswift6" branch of RxSwift, instead of using the 6.0.0 release, so opening the xcodeproj file would fail, as it wasn't in sync with the actual Project.swift file in the repo. This also has the side affect of static linking RxSwift into RxDataSources, which then causes any parent binary that links both RxSwift and RxDataSources to crash because of the known symbol demangling issue. SwiftPM builds are not affected since the relies on the Package.swift.

[thedavidharris]

This seems to only be failing for Xcode 11.5 and I'm unsure as to why

[freak4pc]

Sorry for the delay! I'm ok with the CI failure. Guaranteeing Xcode 11 support isn't a priority right now.

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