mpv: prepare for swift on darwin

[ylh]

Description of changes

In anticipation of Swift coming to Darwin (#189977), these changes will allow swiftSupport to simply be changed to stdenv.isDarwin when the time comes, without encountering any compile errors.

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandbox = true set in nix.conf? (See Nix manual)
• Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)
  • and/or package tests
  • or, for functions and "core" functionality, tests in lib/tests or pkgs/test
  • made sure NixOS tests are linked to the relevant packages
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• Tested basic functionality of all binary files (usually in ./result/bin/)
• 23.05 Release Notes (or backporting 22.11 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
  • (Release notes changes) Ran nixos/doc/manual/md-to-db.sh to update generated release notes
• Fits CONTRIBUTING.md.

[stephank]

I had a slightly different change prepared: stephank@4ce183c

[Atemu]

#189977 has been merged.

[stephank]

We still have to wait for Swift to land on master, I believe.

Regarding the difference between our changes:

• Does AVFoundation enable something extra? It seemed to build without it, but maybe I missed some piece of functionality in testing.
• In my testing, xcrun was required for the build system to find Swift. Does the Swift-specific stuff build and work for you without it?
• I think it's important we don't pull in the Swift compiler as part of the closure, but just its lib output. Could you verify that's the case in your change?

[Atemu]

@stephank the base can simply be changed to staging.

Since you also have an implementation already (which I've used for a bit actually), you can of course always open your own PR.

[wegank]

Superseded by #214944.