Centralize the asset download steps and set up an extensibility point for the VMR just like we have for dotnet/sdk

Author: jkoritzinsky

Directory.Build.props

@@ -242,6 +242,33 @@
     <ArchiveExtension Condition="'$(TargetOsName)' == 'win'">.zip</ArchiveExtension>
   </PropertyGroup>
 
+  <PropertyGroup>
+    <OfficialBaseURL>https://dotnetcli.azureedge.net/dotnet/</OfficialBaseURL>
+    <!-- Allow overriding the public base URL for Unified Build scenarios to pull assets from a local build. -->
+    <PublicBaseURL Condition="'$(PublicBaseURL)' == ''">https://dotnetbuilds.azureedge.net/public/</PublicBaseURL>
+    <InternalBaseURL>https://dotnetbuilds.azureedge.net/internal/</InternalBaseURL>
+  </PropertyGroup>
+
+  <!-- Try various places to find the runtime. It's either released (use official version),
+        public but un-released (use dotnetbuilds/public), or internal and unreleased (use dotnetbuilds/internal) -->
+  <ItemGroup Condition="'$(DotNetBuild) ' != 'true'">
+    <RemoteAssetBaseURL Include="$(OfficialBaseURL)" />
+    <RemoteAssetBaseURL Include="$(PublicBaseURL)" />
+    <!-- Include the token here as we'll generate the URLs to download based on this item group. -->
+    <RemoteAssetBaseURL Include="$(InternalBaseURL)"
+                        Condition=" '$(DotnetRuntimeSourceFeedKey)' != '' ">
+      <token>$(DotnetRuntimeSourceFeedKey)</token>
+    </RemoteAssetBaseURL>
+  </ItemGroup>
+
+  <!--
+    Only try downloading from the "public" base URL when doing a vertical build.
+    In a vertical build, the public URL will be overwritten to point to local build artifacts.
+  -->
+  <ItemGroup Condition="'$(DotNetBuild)' == 'true'">
+    <RemoteAssetBaseURL Include="$(PublicBaseURL)" />
+  </ItemGroup>
+
   <PropertyGroup>
     <!-- PackageReadmeFile specifies the package readme file name in the package. PackageReadmeFilePath points to the package readme file on disk. -->
     <EnableDefaultPackageReadmeFile Condition="'$(EnableDefaultPackageReadmeFile)' == '' and '$(IsShipping)' != 'false'">true</EnableDefaultPackageReadmeFile>

src/Framework/App.Runtime/src/Microsoft.AspNetCore.App.Runtime.csproj

@@ -556,16 +556,9 @@ This package is an internal implementation of the .NET Core SDK and is not meant
   Targets related to creating .zip/.tar.gz
   #########################################
   -->
-  <Target Name="_DownloadAndExtractDotNetRuntime" Condition="'$(DotNetBuild)' != 'true'">
-    <!-- Try various places to find the runtime. It's either released (use official version),
-         public but un-released (use dotnetbuilds/public), or internal and unreleased (use dotnetbuilds/internal) -->
+  <Target Name="_DownloadAndExtractDotNetRuntime">
     <ItemGroup>
-      <UrisToDownload Include="https://dotnetcli.azureedge.net/dotnet/$(DotNetRuntimeDownloadPath)" />
-      <UrisToDownload Include="https://dotnetbuilds.azureedge.net/public/$(DotNetRuntimeDownloadPath)" />
-      <UrisToDownload Include="https://dotnetbuilds.azureedge.net/internal/$(DotNetRuntimeDownloadPath)"
-          Condition=" '$(DotnetRuntimeSourceFeedKey)' != '' ">
-        <token>$(DotnetRuntimeSourceFeedKey)</token>
-      </UrisToDownload>
+      <UrisToDownload Include="@(RemoteAssetBaseURL->'%(Identity)/$(DotNetRuntimeDownloadPath)')" />
     </ItemGroup>
 
     <DownloadFile Condition=" !Exists('$(DotNetRuntimeArchive)') "

src/Installers/Windows/WindowsHostingBundle/Product.targets

@@ -83,12 +83,7 @@
     -->
     <ItemGroup>
       <UrisToDownload Remove="@(UrisToDownload)" />
-      <UrisToDownload Include="https://dotnetcli.azureedge.net/dotnet/Runtime/%(RemoteAsset.Identity)" />
-      <UrisToDownload Include="https://dotnetbuilds.azureedge.net/public/Runtime/%(RemoteAsset.Identity)" />
-      <UrisToDownload Include="https://dotnetbuilds.azureedge.net/internal/Runtime/%(RemoteAsset.Identity)"
-          Condition=" '$(DotnetRuntimeSourceFeedKey)' != '' ">
-        <token>$(DotnetRuntimeSourceFeedKey)</token>
-      </UrisToDownload>
+      <UrisToDownload Include="@(RemoteAssetBaseURL->'%(Identity)/Runtime/%(RemoteAsset.Identity)')" />
     </ItemGroup>
 
     <DownloadFile Condition=" ! Exists('$(DepsPath)%(RemoteAsset.TargetFilename)') "