Add support for extending package metadata item

This allows arbitrary additional metadata to be added to the package metadata
item for other purpose.

Fixes NuGet/Home#5387

Author: kzu

src/Build/NuGet.Build.Packaging.Tasks/NuGet.Build.Packaging.targets

@@ -188,7 +188,8 @@ Copyright (c) .NET Foundation. All rights reserved.
 	<PropertyGroup>
 		<GetPackageContentsDependsOn>
 			$(GetPackageContentsDependsOn);
-			GetPackageTargetPath
+			GetPackageTargetPath;
+			_AddPackageManifest
 		</GetPackageContentsDependsOn>
 		<GetPackageContentsDependsOn Condition="'$(IncludeProjectReferencesInPackage)' == 'true'">
 			$(GetPackageContentsDependsOn);
@@ -197,15 +198,7 @@ Copyright (c) .NET Foundation. All rights reserved.
 	</PropertyGroup>
 
 	<Target Name="GetPackageContents" DependsOnTargets="$(GetPackageContentsDependsOn)" Returns="@(_PackageContent)">
-		<!-- If packaging the project, provide the metadata as a non-file item -->
 		<ItemGroup>
-			<PackageFile Include="@(PackageTargetPath->'%(Id)')" Condition="'$(IsPackable)' == 'true'">
-				<Kind>Metadata</Kind>
-				<PackageId>$(PackageId)</PackageId>
-				<Platform>$(Platform)</Platform>
-				<TargetFrameworkMoniker Condition="'$(IsPackagingProject)' != 'true'">$(TargetFrameworkMoniker)</TargetFrameworkMoniker>
-			</PackageFile>
-
 			<!-- 
 				PackageId metadata on all PackageFile items means we can tell appart which ones came from which dependencies 
 				NOTE: if PackageId is empty, we won't generate a manifest and it means the files need to be packed with the
@@ -225,6 +218,18 @@ Copyright (c) .NET Foundation. All rights reserved.
 		</AssignPackagePath>
 	</Target>
 
+	<Target Name="_AddPackageManifest" Condition="'$(IsPackable)' == 'true'">
+		<!-- If packaging the project, provide the metadata as a non-file item -->
+		<ItemGroup>
+			<PackageFile Include="@(PackageTargetPath->'%(Id)')" Condition="'$(IsPackable)' == 'true'">
+				<Kind>Metadata</Kind>
+				<PackageId>$(PackageId)</PackageId>
+				<Platform>$(Platform)</Platform>
+				<TargetFrameworkMoniker Condition="'$(IsPackagingProject)' != 'true'">$(TargetFrameworkMoniker)</TargetFrameworkMoniker>
+			</PackageFile>
+		</ItemGroup>
+	</Target>
+
 	<Target Name="_GetReferencedPackageContents"
 			Condition="'$(_SupportsProjectReferences)' == 'true'"
 			DependsOnTargets="$(_GetReferencedPackageContentsDependsOn)" 

src/Build/NuGet.Build.Packaging.Tests/Scenarios/given_a_packaging_project/a.nuproj

@@ -22,5 +22,12 @@
 			<Name>e</Name>
 		</ProjectReference>
 	</ItemGroup>
+	<Target Name="_AugmentMetadata" BeforeTargets="GetPackageContents">
+		<ItemGroup>
+			<PackageFile Condition="'%(PackageFile.Kind)' == 'Metadata'">
+				<Foo>Bar</Foo>
+			</PackageFile>
+		</ItemGroup>
+	</Target>
 	<Import Project="$([MSBuild]::GetDirectoryNameOfFileAbove($(MSBuildThisFileDirectory), Scenario.targets))\Scenario.targets" />	
 </Project>

src/Build/NuGet.Build.Packaging.Tests/given_a_packaging_project.cs

@@ -40,6 +40,20 @@ public void when_getting_contents_then_includes_referenced_project_outputs()
 			}));
 		}
 
+		[Fact]
+		public void when_getting_contents_then_can_augment_package_metadata()
+		{
+			var result = Builder.BuildScenario(nameof(given_a_packaging_project), target: "GetPackageContents", output: output);
+
+			result.AssertSuccess(output);
+
+			Assert.Contains(result.Items, item => item.Matches(new
+			{
+				Kind = "Metadata",
+				Foo = "Bar",
+			}));
+		}
+
 		[Fact]
 		public void when_getting_contents_then_includes_referenced_project_satellite_assembly()
 		{