Move the NuGet package build files into a TFM specific directory. (dotnet#370)

When installing Microsoft.ML on an unsupported framework (like net452), it is currently getting installed successfully. However, users should be getting an error stating that net452 is not supported by this package.

The cause is the build files exist for any TFM, which NuGet interprets as this package supports any TFM. Moving the build files to be consistent with the 'lib' folder support.

Fix dotnet#357

Author: eerhardt

Microsoft.ML.sln

@@ -106,6 +106,14 @@ Project("{9A19103F-16F7-4668-BE54-9A1E7A4F7556}") = "Microsoft.ML.Maml", "src\Mi
 EndProject
 Project("{9A19103F-16F7-4668-BE54-9A1E7A4F7556}") = "Microsoft.ML.Console", "src\Microsoft.ML.Console\Microsoft.ML.Console.csproj", "{362A98CF-FBF7-4EBB-A11B-990BBF845B15}"
 EndProject
+Project("{2150E333-8FDC-42A3-9474-1A3956D46DE8}") = "build", "build", "{487213C9-E8A9-4F94-85D7-28A05DBBFE3A}"
+EndProject
+Project("{2150E333-8FDC-42A3-9474-1A3956D46DE8}") = "netstandard2.0", "netstandard2.0", "{9252A8EB-ABFB-440C-AB4D-1D562753CE0F}"
+	ProjectSection(SolutionItems) = preProject
+		pkg\Microsoft.ML\build\netstandard2.0\Microsoft.ML.props = pkg\Microsoft.ML\build\netstandard2.0\Microsoft.ML.props
+		pkg\Microsoft.ML\build\netstandard2.0\Microsoft.ML.targets = pkg\Microsoft.ML\build\netstandard2.0\Microsoft.ML.targets
+	EndProjectSection
+EndProject
 Global
 	GlobalSection(SolutionConfigurationPlatforms) = preSolution
 		Debug|Any CPU = Debug|Any CPU
@@ -243,6 +251,8 @@ Global
 		{7A9DB75F-2CA5-4184-9EF5-1F17EB39483F} = {AED9C836-31E3-4F3F-8ABC-929555D3F3C4}
 		{64F40A0D-D4C2-4AA7-8470-E9CC437827E4} = {09EADF06-BE25-4228-AB53-95AE3E15B530}
 		{362A98CF-FBF7-4EBB-A11B-990BBF845B15} = {09EADF06-BE25-4228-AB53-95AE3E15B530}
+		{487213C9-E8A9-4F94-85D7-28A05DBBFE3A} = {DEC8F776-49F7-4D87-836C-FE4DC057D08C}
+		{9252A8EB-ABFB-440C-AB4D-1D562753CE0F} = {487213C9-E8A9-4F94-85D7-28A05DBBFE3A}
 	EndGlobalSection
 	GlobalSection(ExtensibilityGlobals) = postSolution
 		SolutionGuid = {41165AF1-35BB-4832-A189-73060F82B01D}

pkg/Microsoft.ML/build/Microsoft.ML.props renamed to pkg/Microsoft.ML/build/netstandard2.0/Microsoft.ML.props

@@ -7,7 +7,7 @@
   <ItemGroup Condition="Exists('packages.config') OR
                         Exists('$(MSBuildProjectName).packages.config') OR
                         Exists('packages.$(MSBuildProjectName).config')">
-    <Content Include="$(MSBuildThisFileDirectory)\..\runtimes\win-x64\native\*.dll"
+    <Content Include="$(MSBuildThisFileDirectory)\..\..\runtimes\win-x64\native\*.dll"
              Condition="'$(PlatformTarget)' == 'x64'">
       <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
       <Visible>false</Visible>