update podspec

Author: Andrey Yastrebov

DiffableDataSources.podspec

@@ -8,7 +8,6 @@ Pod::Spec.new do |spec|
   spec.source = { :git => 'https://github.com/ra1028/DiffableDataSources.git', :tag => spec.version.to_s }
   spec.license = { :type => 'Apache 2.0', :file => 'LICENSE' }
   spec.requires_arc = true
-  spec.source_files = 'Sources/**/*.swift'
   spec.swift_versions = ["5.0", "5.1"]
 
   differenekit_version = '~> 1.1'
@@ -17,11 +16,16 @@ Pod::Spec.new do |spec|
   spec.tvos.dependency 'DifferenceKit/UIKitExtension', differenekit_version
   spec.osx.dependency 'DifferenceKit/AppKitExtension', differenekit_version
 
-  spec.ios.frameworks = 'UIKit'
+  spec.source_files       = 'Sources/Internal/*.swift', 'Sources/*.swift'
+  spec.ios.source_files   = 'Sources/UIKit/*.swift'
+  spec.tvos.source_files  = 'Sources/UIKit/*.swift'
+  spec.osx.source_files   = 'Sources/AppKit/*.swift'
+
+  spec.ios.frameworks  = 'UIKit'
   spec.tvos.frameworks = 'UIKit'
-  spec.osx.frameworks = 'Appkit'
+  spec.osx.frameworks  = 'Appkit'
 
-  spec.ios.deployment_target = '9.0'
+  spec.ios.deployment_target  = '9.0'
   spec.tvos.deployment_target = '9.0'
-  spec.osx.deployment_target = '10.11'
+  spec.osx.deployment_target  = '10.11'
 end