Implement fan-out behavior for publish.

Contributes to #22.

src/sdk/build/Zig.Sdk.Build.targets

@@ -1,7 +1,7 @@
 <Project>
     <PropertyGroup>
         <_RealOrFanOutBuild>_InnerBuild</_RealOrFanOutBuild>
-        <_RealOrFanOutBuild Condition="'$(_BuildCleanFanOut)' == 'true'">_FanOutBuild</_RealOrFanOutBuild>
+        <_RealOrFanOutBuild Condition="'$(_RidFanOut)' == 'true'">_FanOutBuild</_RealOrFanOutBuild>
     </PropertyGroup>
 
     <Target Name="Build"

src/sdk/build/Zig.Sdk.Clean.targets

@@ -10,7 +10,7 @@
 
     <PropertyGroup>
         <_RealOrFanOutClean>_InnerClean</_RealOrFanOutClean>
-        <_RealOrFanOutClean Condition="'$(_BuildCleanFanOut)' == 'true'">_FanOutClean</_RealOrFanOutClean>
+        <_RealOrFanOutClean Condition="'$(_RidFanOut)' == 'true'">_FanOutClean</_RealOrFanOutClean>
     </PropertyGroup>
 
     <Target Name="Clean"

src/sdk/build/Zig.Sdk.Overrides.targets

@@ -17,14 +17,14 @@
     </PropertyGroup>
 
     <!--
-    If a RuntimeIdentifier value has not been set, it means the Build and Clean
-    targets should fan out and run for every RID supported by the project. That
-    said, for running and testing, we still need to run natively by default, so
-    we still have to set a default RID. We use _BuildCleanFanOut to indicate
-    whether the fan-out behavior is required.
+    If a RuntimeIdentifier value has not been set, it means the Build, Clean,
+    and Publish targets should fan out and run for every RID supported by the
+    project. That said, for running and testing, we still need to run natively
+    by default, so we still have to set a default RID. We use _RidFanOut to
+    indicate whether the fan-out behavior is required.
     -->
     <PropertyGroup Condition="'$(RuntimeIdentifier)' == ''">
         <RuntimeIdentifier>$(NETCoreSdkPortableRuntimeIdentifier)</RuntimeIdentifier>
-        <_BuildCleanFanOut>true</_BuildCleanFanOut>
+        <_RidFanOut>true</_RidFanOut>
     </PropertyGroup>
 </Project>

src/sdk/build/Zig.Sdk.Publish.targets

@@ -1,3 +1,38 @@
 <Project>
+    <PropertyGroup>
+        <_RealOrFanOutPublish>_InnerPublish</_RealOrFanOutPublish>
+        <_RealOrFanOutPublish Condition="'$(_RidFanOut)' == 'true'">_FanOutPublish</_RealOrFanOutPublish>
+    </PropertyGroup>
+
+    <Target Name="Publish"
+            DependsOnTargets="$(_RealOrFanOutPublish)"
+            Condition="'$(_InvalidConfigurationWarning)' != 'true'" />
+
+    <Target Name="_FanOutPublish">
+        <ItemGroup>
+            <_RuntimeIdentifiers Include="$(RuntimeIdentifiers)" />
+            <_InnerProjects Include="$(MSBuildProjectFullPath)"
+                            Properties="RuntimeIdentifier=%(_RuntimeIdentifiers.Identity)" />
+        </ItemGroup>
+
+        <MSBuild Projects="@(_InnerProjects)"
+                 Targets="_InnerPublish"
+                 BuildInParallel="$(BuildInParallel)" />
+    </Target>
+
+    <!--
+    Unfortunately, the publish logic in the .NET SDK is not factored as well as
+    the build and clean logic in MSBuild, so we have to duplicate some of the
+    logic in the default Publish target here.
+    -->
+    <Target Name="_InnerPublish"
+            DependsOnTargets="_PublishBuildAlternative; _PublishNoBuildAlternative">
+        <Message Text="$(MSBuildProjectName) -> $([System.IO.Path]::GetFullPath('$(PublishDir)'))"
+                 Importance="high" />
+
+        <AllowEmptyTelemetry EventName="PublishProperties"
+                             EventData="PublishReadyToRun=$(PublishReadyToRun); PublishTrimmed=$(PublishTrimmed); PublishSingleFile=$(PublishSingleFile)" />
+    </Target>
+
     <!-- TODO: https://github.com/alexrp/zig-msbuild-sdk/issues/16 -->
 </Project>