Change mechanism for TFM support warnings

Apparently when we tried to pack a TFM-specific file called _._ into
buildTransitive to indicate that there were no targets for that TFM,
NuGet arbitrarily decided to pack the targets file from a different TFM
instead ¯\_(ツ)_/¯

Borrow the logic from https://github.com/dotnet/runtime/blob/bb74bb79db53eae474977a0928756a0db7fc6a3f/src/libraries/Microsoft.Extensions.Configuration.Binder/src/buildTransitive/Microsoft.Extensions.Configuration.Binder.targets#L17
as it's easier than overriding NuGet's weird pack behavior.

Fixes #174

Mono.TextTemplating/Mono.TextTemplating.csproj

@@ -24,10 +24,7 @@
     <!-- limit to 6.0 so that the MSBuild task can load on .NET 6.0 -->
     <PackageReference Include="System.CodeDom" Version="6.0.0" Condition="'$(TargetFramework)'!='net472'" />
     <None Include="readme.md" Pack="true" PackagePath="\" />
-    <None Include="package\Mono.TextTemplating.targets" Pack="true" PackagePath="buildTransitive\net461\Mono.TextTemplating.targets" />
-    <None Include="package\_._" Pack="true" PackagePath="buildTransitive\net472\_._" />
-    <None Include="package\Mono.TextTemplating.targets" Pack="true" PackagePath="buildTransitive\netcoreapp2.0\Mono.TextTemplating.targets" />
-    <None Include="package\_._" Pack="true" PackagePath="buildTransitive\net6.0\_._" />
+    <None Include="package\Mono.TextTemplating.targets" Pack="true" PackagePath="buildTransitive\Mono.TextTemplating.targets" />
   </ItemGroup>
 
   <ItemGroup>

Mono.TextTemplating/package/Mono.TextTemplating.targets

@@ -1,6 +1,18 @@
 <Project InitialTargets="NETStandardCompatError_Mono_TextTemplating_net6_0">
   <Target Name="NETStandardCompatError_Mono_TextTemplating_net6_0"
           Condition="'$(SuppressTfmSupportBuildWarnings)' == ''">
-    <Warning Text="Mono_TextTemplating doesn't support $(TargetFramework) and has not been tested with it. Consider upgrading your TargetFramework to net6.0 or later. You may also set &lt;SuppressTfmSupportBuildWarnings&gt;true&lt;/SuppressTfmSupportBuildWarnings&gt; in the project file to ignore this warning and attempt to run in this unsupported configuration at your own risk." />
+    <PropertyGroup>
+      <_Mono_TextTemplating_Warn_Compat_Tfm
+          Condition="$([MSBuild]::IsTargetFrameworkCompatible('$(TargetFramework)', 'netcoreapp2.0')) AND
+          !$([MSBuild]::IsTargetFrameworkCompatible('$(TargetFramework)', 'net6.0'))">
+      >net6.0</_Mono_TextTemplating_Warn_Compat_Tfm>
+      <_Mono_TextTemplating_Warn_Compat_Tfm
+          Condition="$([MSBuild]::IsTargetFrameworkCompatible('$(TargetFramework)', 'net461')) AND
+                     !$([MSBuild]::IsTargetFrameworkCompatible('$(TargetFramework)', 'net462'))"
+      >net462</_Mono_TextTemplating_Warn_Compat_Tfm>
+    </PropertyGroup>
+    <Warning
+        Condition="'$(_Mono_TextTemplating_Warn_Compat_Tfm)' != ''"
+        Text="Mono_TextTemplating doesn't support $(TargetFramework) and has not been tested with it. Consider upgrading your TargetFramework to $(_Mono_TextTemplating_Warn_Compat_Tfm) or later. You may also set &lt;SuppressTfmSupportBuildWarnings&gt;true&lt;/SuppressTfmSupportBuildWarnings&gt; in the project file to ignore this warning and attempt to run in this unsupported configuration at your own risk." />
   </Target>
 </Project>