Add Swift Package Manager support

[zbarbuto]

Closes #980
Closes #1014

• Add Package.swift
• Move resources and source files to required directory for SPM
• Update podspec to use SPM directory for resources
• Load database init file from location depending on package manager

The package now supports both CocoaPods and SPM. You can test the example by editing pubspec.yaml and toggling flutter -> config -> enable-swift-package-manager. If it is disabled a Podfile should be created automatically - though you may need to make some tweaks to min iOS version and remove the tests.

I mostly followed the official guide for this but for a few things I relied on a similar PR for permission handler.

Example Podfile

You may need to revert some of the changes to the .xcodeproj .xcodeworkspace. as well for this to work.

# Uncomment this line to define a global platform for your project
platform :ios, '13.0'

# CocoaPods analytics sends network stats synchronously affecting flutter build latency.
ENV['COCOAPODS_DISABLE_STATS'] = 'true'

project 'Runner', {
  'Debug' => :debug,
  'Profile' => :release,
  'Release' => :release,
}

def flutter_root
  generated_xcode_build_settings_path = File.expand_path(File.join('..', 'Flutter', 'Generated.xcconfig'), __FILE__)
  unless File.exist?(generated_xcode_build_settings_path)
    raise "#{generated_xcode_build_settings_path} must exist. If you're running pod install manually, make sure flutter pub get is executed first"
  end

  File.foreach(generated_xcode_build_settings_path) do |line|
    matches = line.match(/FLUTTER_ROOT\=(.*)/)
    return matches[1].strip if matches
  end
  raise "FLUTTER_ROOT not found in #{generated_xcode_build_settings_path}. Try deleting Generated.xcconfig, then run flutter pub get"
end

require File.expand_path(File.join('packages', 'flutter_tools', 'bin', 'podhelper'), flutter_root)

flutter_ios_podfile_setup

target 'Runner' do
  use_frameworks!

  flutter_install_all_ios_pods File.dirname(File.realpath(__FILE__))
end

post_install do |installer|
  installer.pods_project.targets.each do |target|
    flutter_additional_ios_build_settings(target)
  end
end