[release/8.0-preview1] Split Mono manifest creation into a separate subset (#81719)

Backport of https://github.com/dotnet/runtime/pull/81598

Fix up #80920 so building manifests works when the runtime is CoreCLR (as is typically the case on source-build)
Co-authored-by: NJo Shields <directhex@apebox.org>

eng/Subsets.props

@@ -64,6 +64,7 @@
     <DefaultMonoSubsets Condition="'$(TargetOS)' == 'wasi'">$(DefaultMonoSubsets)mono.wasiruntime+</DefaultMonoSubsets>
     <DefaultMonoSubsets Condition="'$(MonoCrossAOTTargetOS)' != ''">$(DefaultMonoSubsets)mono.aotcross+</DefaultMonoSubsets>
     <DefaultMonoSubsets>$(DefaultMonoSubsets)mono.runtime+mono.corelib+mono.packages+</DefaultMonoSubsets>
+    <DefaultMonoSubsets Condition="'$(TargetsMobile)' == 'true' or '$(ForceBuildMobileManifests)' == 'true'">$(DefaultMonoSubsets)mono.manifests+</DefaultMonoSubsets>
     <DefaultMonoSubsets Condition="'$(PrimaryRuntimeFlavor)' != 'Mono'">$(DefaultMonoSubsets)mono.tools+</DefaultMonoSubsets>
     <DefaultMonoSubsets Condition="'$(TargetsMobile)' != 'true'">$(DefaultMonoSubsets)host.native+</DefaultMonoSubsets>
 
@@ -82,6 +83,7 @@
     <DefaultPacksSubsets>packs.product</DefaultPacksSubsets>
     <DefaultPacksSubsets Condition="'$(BuildMonoAOTCrossCompilerOnly)' != 'true' and '$(DotNetBuildFromSource)' != 'true'">$(DefaultPacksSubsets)+packs.tests</DefaultPacksSubsets>
     <DefaultPacksSubsets Condition="'$(DotNetBuildFromSource)' == 'true'">$(DefaultPacksSubsets)+packs.installers</DefaultPacksSubsets>
+    <DefaultPacksSubsets Condition="'$(RuntimeFlavor)' != 'Mono' and '$(ForceBuildMobileManifests)' == 'true'">$(DefaultPacksSubsets)+mono.manifests</DefaultPacksSubsets>
   </PropertyGroup>
 
   <PropertyGroup>
@@ -141,6 +143,7 @@
     <SubsetName Include="Mono.Runtime" Description="The Mono .NET runtime." />
     <SubsetName Include="Mono.AotCross" Description="The cross-compiler runtime for Mono AOT." />
     <SubsetName Include="Mono.CoreLib" Description="The managed System.Private.CoreLib library for Mono." />
+    <SubsetName Include="Mono.Manifests" Description="The NuGet packages with manifests defining the mobile and Blazor workloads." />
     <SubsetName Include="Mono.Packages" Description="The projects that produce NuGet packages for the Mono runtime." />
     <SubsetName Include="Mono.Tools" Description="Tooling that helps support Mono development and testing." />
     <SubsetName Include="Mono.WasmRuntime" Description="The Emscripten runtime." />
@@ -382,6 +385,10 @@
     <ProjectToBuild Include="$(MonoProjectRoot)llvm\llvm-init.proj" Category="mono" />
   </ItemGroup>
 
+  <ItemGroup Condition="$(_subset.Contains('+mono.manifests+'))">
+    <ProjectToBuild Include="$(MonoProjectRoot)nuget\manifest-packages.proj" Category="mono" Pack="true" />
+  </ItemGroup>
+
   <ItemGroup Condition="$(_subset.Contains('+mono.packages+'))">
     <ProjectToBuild Include="$(MonoProjectRoot)nuget\mono-packages.proj" Category="mono" Pack="true" />
   </ItemGroup>

src/mono/nuget/manifest-packages.proj

+<Project Sdk="Microsoft.Build.Traversal" DefaultTargets="Pack">
+  <ItemGroup>
+    <ProjectReference Include="Microsoft.NET.Workload.Mono.Toolchain.Current.Manifest\Microsoft.NET.Workload.Mono.Toolchain.Current.Manifest.pkgproj" />
+    <ProjectReference Include="Microsoft.NET.Workload.Mono.Toolchain.net6.Manifest\Microsoft.NET.Workload.Mono.Toolchain.net6.Manifest.pkgproj" />
+    <ProjectReference Include="Microsoft.NET.Workload.Mono.Toolchain.net7.Manifest\Microsoft.NET.Workload.Mono.Toolchain.net7.Manifest.pkgproj" />
+  </ItemGroup>
+</Project>

src/mono/nuget/mono-packages.proj

@@ -14,10 +14,7 @@
     <ProjectReference Include="Microsoft.NET.Runtime.iOS.Sample.Mono\Microsoft.NET.Runtime.iOS.Sample.Mono.pkgproj" />
   </ItemGroup>
 
-  <ItemGroup Condition="'$(TargetsMobile)' == 'true' or '$(ForceBuildMobileManifests)' == 'true'">
-    <ProjectReference Include="Microsoft.NET.Workload.Mono.Toolchain.Current.Manifest\Microsoft.NET.Workload.Mono.Toolchain.Current.Manifest.pkgproj" />
-    <ProjectReference Include="Microsoft.NET.Workload.Mono.Toolchain.net6.Manifest\Microsoft.NET.Workload.Mono.Toolchain.net6.Manifest.pkgproj" />
-    <ProjectReference Include="Microsoft.NET.Workload.Mono.Toolchain.net7.Manifest\Microsoft.NET.Workload.Mono.Toolchain.net7.Manifest.pkgproj" />
+  <ItemGroup Condition="'$(TargetsMobile)' == 'true'">
     <ProjectReference Include="Microsoft.NET.Runtime.MonoAOTCompiler.Task\Microsoft.NET.Runtime.MonoAOTCompiler.Task.pkgproj" />
   </ItemGroup>