[Xamarin.Android.Build.Tasks] ILRepack the NuGet.* Tasks Dependencies. (

dotnet#1694)

`<ResolveAssemblies/>` is throwing the following error on a newer
mono versions (2018-04):

	error : Could not load file or assembly 'NuGet.ProjectModel, Version=4.6.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35' or one of its dependencies.

This is because we took a dependency on NuGet.ProjectModel 4.6 and
the newer mono has 4.7.

We cannot always be sure which version of the NuGet assemblies will
be installed since users are able to upgrade mono independently.
Instead, use the `ILRepack` NuGet to package up the dependencies we
need so that `Xamarin.Android.Build.Tasks.dll` contains all the
correct assemblies it needs.

src/Xamarin.Android.Build.Tasks/ILRepack.targets

@@ -0,0 +1,29 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<Project xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
+	<Target Name="ILRepacker" AfterTargets="Build">
+		<ItemGroup>
+			<InputAssemblies Include="$(OutputPath)\NuGet.ProjectModel.dll" />
+			<InputAssemblies Include="$(OutputPath)\Newtonsoft.Json.dll" />
+			<InputAssemblies Include="$(OutputPath)\NuGet.Common.dll" />
+			<InputAssemblies Include="$(OutputPath)\NuGet.Common.dll" />
+			<InputAssemblies Include="$(OutputPath)\NuGet.Frameworks.dll" />
+			<InputAssemblies Include="$(OutputPath)\NuGet.LibraryModel.dll" />
+			<InputAssemblies Include="$(OutputPath)\NuGet.Versioning.dll" />
+			<InputAssemblies Include="$(OutputPath)\NuGet.Packaging.dll" />
+			<InputAssemblies Include="$(OutputPath)\NuGet.Packaging.Core.dll" />
+			<InputAssemblies Include="$(OutputPath)\NuGet.DependencyResolver.Core.dll" />
+			<InputAssemblies Include="$(OutputPath)\NuGet.Configuration.dll" />
+		</ItemGroup>
+		<ILRepack Parallel="true"
+			Internalize="true"
+			Verbose="true"
+			DebugInfo="true"
+			InternalizeExclude="@(DoNotInternalizeAssemblies)"
+			InputAssemblies="$(OutputPath)\$(AssemblyName).dll;@(InputAssemblies)"
+			LibraryPath="$(OutputPath)"
+			TargetKind="Dll"
+			OutputFile="$(OutputPath)\$(AssemblyName).dll"
+		/>
+		<Delete Files="@(InputAssemblies)" />
+	</Target>
+</Project>

src/Xamarin.Android.Build.Tasks/Xamarin.Android.Build.Tasks.csproj

@@ -585,6 +585,7 @@
   </ItemGroup>
   <ItemGroup>
     <None Include="Xamarin.Android.Build.Tasks.targets" />
+    <None Include="ILRepack.targets" />
     <None Include="packages.config" />
   </ItemGroup>
   <ItemGroup>
@@ -733,4 +734,5 @@
     <Folder Include="Linker\Linker\" />
     <Folder Include="utils\" />
   </ItemGroup>
+  <Import Project="..\..\packages\ILRepack.Lib.MSBuild.Task.2.0.15.4\build\ILRepack.Lib.MSBuild.Task.targets" Condition="Exists('..\..\packages\ILRepack.Lib.MSBuild.Task.2.0.15.4\build\ILRepack.Lib.MSBuild.Task.targets')" />
 </Project>

src/Xamarin.Android.Build.Tasks/packages.config

@@ -16,4 +16,5 @@
   <package id="NuGet.Versioning" version="4.6.0" targetFramework="net462" />
   <package id="System.Runtime" version="4.3.0" targetFramework="net462" />
   <package id="System.Runtime.InteropServices" version="4.3.0" targetFramework="net462" />
-</packages>
+  <package id="ILRepack.Lib.MSBuild.Task" version="2.0.15.4" targetFramework="net462" />
+</packages>