WIP feat!: SPM support

[jcesarmobile]

This is just a test for converting the plugin to a Swift Package Manager so apache/cordova-ios#1430 can be tested with something.
I had a local copy that didn't have the privacy manifest so I didn't add that to the Package.swift, if we go with this approach we should add it.

It removes header-file and source-file to prevent cordova plugin add from adding those files to the cordova app since they will be added by Swift Package Manager.

Using this approach plugins that want to add Swift Package Manager dependencies will have to be converted to be Swift Package Manager compatible and add dependences as they would to a regular Swift Package instead of relying in a new tag in plugin.xml

It's a breaking change because if converted, it will only work in the cordova-ios version that adds SPM plugins support (apache/cordova-ios#1430) since this change removes the header-file and source-file.
Maybe apache/cordova-ios#1430 could be tweaked to conditionally respect the header-file and source-file, etc. tags and copy them only if there is no Package.swift in the plugin, but would require a ton more work.

[dpogue]

This could potentially be an issue since we can't distribute stuff pointing to the development branch, and it's probably bad if this doesn't match the version of CordovaLib being used in the app.

I guess SwiftPM has no concept of anything like "peer dependencies"? 😕

[jcesarmobile]

I've removed it since by keeping CordovaLib in the app project, having the dependency is not needed.

We will miss having plugins that are "standalone" so the source code can be built with xcodebuild without adding them to an app, but that is not possible at the moment neither, so not a big miss.