build: SPM temp support changes for support on MacOS

[Xa69]

This PR is for temporary support for building with SPM instead of Cocoapods.

This change is only going to be merged until the plugin is migrated to use a shared /darwin directory for both iOS and macOS.

PR created on the request from another PR for iOS.
Reference PR : #1155

[navaronbracke]

One nit about a blank line, but otherwise this looks good!

[navaronbracke]

@Xa69 Small question, according to Flutter's migration guide, it says "Flutter falls back to CocoaPods for dependencies that do not support Swift Package Manager yet."

Does this mean that, when SPM support is disabled for the Flutter CLI / this plugin, the Cocoapods version still works (because the sources are now just in a different directory, but otherwise unchanged?)

If it still works for Cocoapods, we can go ahead and land SPM support for MacOS already. It will only be widely available (and turned on) when SPM support is available on stable anyway.

[Xa69]

@navaronbracke yes it will work even if the SPM config flag is disabled, I’ve observed it in the build step it will install cocoapods dependencies if the flag "—no-enable-swift-package-manager” is set. [This flag will set it back to use Cocoapods instead of SPM]

This confirms that it will work with cocoapods.

[Xa69]

@navaronbracke Looks like only people with ‘Write’ access to the main repository can merge this PR, I don’t have an option to complete it.

@juliansteenbakker If possible, please review the PR and merge the code please.

[navaronbracke]

As adding SPM support won't affect our users that are using Cocoapods, per the note in the migration guide, I'll merge this already.