Here's one way you could do it. Include the following into your scproj file (or in an imported .targets file). (Note: the following assumes only one HedgehogDevelopment.TDS NuGet package folder, and that your custom validators are in a 'MyExtensions.TDS.CustomExtensions' NuGet package and DLL).

  <UsingTask TaskName="GetTdsNuGetPackageFolder" TaskFactory="CodeTaskFactory" AssemblyFile="$(MSBuildToolsPath)\Microsoft.Build.Tasks.v4.0.dll">
    <ParameterGroup>
      <PackagesFolderPath ParameterType="System.String" Required="true" />
      <TdsNuGetFolder ParameterType="System.String" Output="true" />
    </ParameterGroup>
    <Task>
      <Using Namespace="System" />
      <Using Namespace="System.IO" />
      <Using Namespace="System.Text.RegularExpressions" />
      <Code Type="Fragment" Language="cs"><![CDATA[
        var hhogTDSNuGetNamePattern = "HedgehogDevelopment.TDS.*" ;
        var hhogTDSPackageFolders = Directory.EnumerateDirectories(PackagesFolderPath, hhogTDSNuGetNamePattern);
        foreach(var folder in hhogTDSPackageFolders)
        {
            var tasksDllFolder = folder + "/build";
            var tasksFileExists = File.Exists(tasksDllFolder + "/HedgehogDevelopment.SitecoreProject.Tasks.dll");
            if (tasksFileExists)
            {
                TdsNuGetFolder = tasksDllFolder;
                return true;
            }
        }
        TdsNuGetFolder = "";
        ]]></Code>
    </Task>
  </UsingTask>

  <Target Name="BeforeSitecoreBuild">
    <GetTdsNuGetPackageFolder PackagesFolderPath="$(SolutionDir)/packages/">
      <Output TaskParameter="TdsNuGetFolder" PropertyName="TdsNuGetPackageDllFolder" />
    </GetTdsNuGetPackageFolder>
    
    <Message Condition="$(TdsNuGetPackageDllFolder) == ''"
             Text="Solution doesn't use the HedgehogDevelopment.TDS NuGet package. You need to manually copy the 'packages/MyExtensions.TDS.CustomExtensions.1.0.0.0/build/MyExtensions.TDS.CustomExtensions.dll' to your MSBuild directory (C:\Program Files (x86)\MSBuild\HedgehogDevelopment\SitecoreProject\v9.0).'" />
    <Message Condition="$(TdsNuGetPackageDllFolder) != ''"
             Text="Solution uses the HedgehogDevelopment.TDS NuGet package. MyExtensions.TDS.CustomExtensions.DLL will load from there." />
    
    <Copy Condition="Exists('$(TdsNuGetPackageDllFolder)/HedgehogDevelopment.SitecoreProject.Tasks.dll') and !Exists('$(TdsNuGetPackageDllFolder)/MyExtensions.TDS.CustomExtensions.dll')"
          SourceFiles="$(SolutionDir)/packages/MyExtensions.TDS.CustomExtensions.1.0.0.0/build/MyExtensions.TDS.CustomExtensions.dll"
          DestinationFolder="$(TdsNuGetPackageDllFolder)/" />
  </Target>