[msbuild] Share the Pack/UnpackLibraryResources targets between Xamarin.iOS and Xamarin.Mac.

[rolfbjarne]

There were 2 differences that had to be reconciled:

• The Xamarin.Mac version took 'IntermediateOutputPath' as the intermediate
  output path, while the Xamarin.iOS version used
  'DeviceSpecificIntermediateOutputPath'.
  'DeviceSpecificIntermediateOutputPath' is defined to be
  'IntermediateOutputPath' for Xamarin.Mac, so make the shared version use
  'DeviceSpecificIntermediateOutputPath', since that works in both cases.

• The Xamarin.Mac version was writing the output of PackLibraryResources to
  the FileWrites item group. This was removed on purpose from the Xamarin.iOS
  version in e97d69b, so it seemed best to
  leave it out in the shared version as well.

Also re-use the _EmbeddedResourcePrefix variable to determine the resource prefix.

[monojenkins]

Build failure
✅ Provisioning succeeded
✅ Build succeeded
✅ API Diff (from stable)
✅ API Diff (from PR only) (no change)
✅ Generator Diff (no change)
🔥 Test run failed 🔥

Test results

2 tests failed, 91 tests passed.

Failed tests

• interdependent-binding-projects/iOS Unified 64-bits - simulator/Debug [dotnet]: BuildFailure (Harness exception for 'interdependent-binding-projects': System.IO.DirectoryNotFoundException: Could not find a part of the path "/Users/builder/jenkins/workspace/xamarin-macios-pr-builder/tests/external/Touch.Unit/Touch.Client/dotnet/iOS/Touch.Client-iOS.dotnet.csproj".
  at System.IO.FileStream..ctor (System.String path, System.IO.FileMode mode, System.IO.FileAccess access, System.IO.FileShare share, System.Int32 bufferSize, System.Boolean anonymous, System.IO.FileOptions options) [0x0015e] in /Users/builder/jenkins/workspace/build-package-osx-mono/2020-02/external/bockbuild/builds/mono-x64/mcs/class/corlib/System.IO/FileStream.cs:223
  at System.IO.FileStream..ctor (System.String path, System.IO.FileMode mode, System.IO.FileAccess access, System.IO.FileShare share, System.Int32 bufferSize, System.Boolean isAsync, System.Boolean anonymous) [0x00000] in /Users/builder/jenkins/workspace/build-package-osx-mono/2020-02/external/bockbuild/builds/mono-x64/mcs/class/corlib/System.IO/FileStream.cs:149
  at System.IO.FileStream..ctor (System.String path, System.IO.FileMode mode, System.IO.FileAccess access) [0x00000] in /Users/builder/jenkins/workspace/build-package-osx-mono/2020-02/external/bockbuild/builds/mono-x64/mcs/class/corlib/System.IO/FileStream.cs:86
  at (wrapper remoting-invoke-with-check) System.IO.FileStream..ctor(string,System.IO.FileMode,System.IO.FileAccess)
  at Microsoft.DotNet.XHarness.iOS.Shared.Utilities.PListExtensions.LoadWithoutNetworkAccess (System.Xml.XmlDocument doc, System.String filename) [0x00001] in /Users/builder/jenkins/workspace/xamarin-macios-pr-builder/tests/xharness/Microsoft.DotNet.XHarness.iOS.Shared/Utilities/PlistExtensions.cs:17
  at Microsoft.DotNet.XHarness.iOS.Shared.TestProject.CreateCopyAsync (Microsoft.DotNet.XHarness.iOS.Shared.Logging.ILog log, Microsoft.DotNet.XHarness.iOS.Shared.Execution.IProcessManager processManager, Microsoft.DotNet.XHarness.iOS.Shared.Tasks.ITestTask test, System.String rootDirectory, System.Collections.Generic.Dictionary2[TKey,TValue] allProjectReferences) [0x0010c] in /Users/builder/jenkins/workspace/xamarin-macios-pr-builder/tests/xharness/Microsoft.DotNet.XHarness.iOS.Shared/TestProject.cs:98 at Microsoft.DotNet.XHarness.iOS.Shared.TestProject.CreateCopyAsync (Microsoft.DotNet.XHarness.iOS.Shared.Logging.ILog log, Microsoft.DotNet.XHarness.iOS.Shared.Execution.IProcessManager processManager, Microsoft.DotNet.XHarness.iOS.Shared.Tasks.ITestTask test, System.String rootDirectory, System.Collections.Generic.Dictionary2[TKey,TValue] allProjectReferences) [0x00811] in /Users/builder/jenkins/workspace/xamarin-macios-pr-builder/tests/xharness/Microsoft.DotNet.XHarness.iOS.Shared/TestProject.cs:168
  at Microsoft.DotNet.XHarness.iOS.Shared.Tasks.TestTasks.RunInternalAsync () [0x00117] in /Users/builder/jenkins/workspace/xamarin-macios-pr-builder/tests/xharness/Microsoft.DotNet.XHarness.iOS.Shared/Tasks/TestTask.cs:274 )
• monotouch-test/watchOS 32-bits - simulator/Debug: Failed Known issue: HE0038)

[rolfbjarne]

Test failure is unrelated, and was fixed in PR #9467.