Force project references to be included in netstandard nuget package - visual-studio-2017

Question

I have a netstandard project which includes two project references. Visual studio 2017 is being used to build the nukpg. When the project is built the produced nupkg only contains the assembly produced by that project and lists the two project references as nuget dependencies. Is there a way to force the packaging to include those assemblies as lib files?
csproj
<Project Sdk="Microsoft.NET.Sdk">
<PropertyGroup>
<TargetFramework>net462</TargetFramework>
<RootNamespace>Verifier.Observations.DevOps.Health</RootNamespace>
<GeneratePackageOnBuild>true</GeneratePackageOnBuild>
<VersionPrefix>1.0.1</VersionPrefix>
</PropertyGroup>
<ItemGroup>
<ProjectReference Include="..\Verifier.Observations.Aspects\Verifier.Observations.Aspects.csproj" />
<ProjectReference Include="..\Verifier.Observations\Verifier.Observations.csproj" />
</ItemGroup>
<ItemGroup>
<Reference Include="System.ComponentModel.Composition"/>
<Reference Include="System.Net.Http" />
</ItemGroup>
</Project>
Update
Based upon feedback from #alexgiondea-msft the package is now created as desired using the following
csproj
<Project Sdk="Microsoft.NET.Sdk">
<PropertyGroup>
<VersionPrefix>1.0.1</VersionPrefix>
<TargetFramework>net462</TargetFramework>
<GeneratePackageOnBuild>true</GeneratePackageOnBuild>
<NuspecFile>Verifier.Observations.DevOps.Health.Nuspec</NuspecFile>
<NuspecProperties>version=$(VersionPrefix);id=$(MSBuildProjectName);author=$(Authors);copy=$(Copyright);iconUrl=$(PackageIconUrl)</NuspecProperties>
</PropertyGroup>
<ItemGroup>
<ProjectReference Include="..\Verifier.Observations.Aspects\Verifier.Observations.Aspects.csproj" />
<ProjectReference Include="..\Verifier.Observations\Verifier.Observations.csproj" />
</ItemGroup>
<ItemGroup>
<Reference Include="System.ComponentModel.Composition" />
<Reference Include="System.Net.Http" />
</ItemGroup>
</Project>
nuspec
<package >
<metadata>
<id>$id$</id>
<version>$version$</version>
<title>$title$</title>
<authors>$author$</authors>
<owners>$author$</owners>
<iconUrl>$iconUrl$</iconUrl>
<requireLicenseAcceptance>false</requireLicenseAcceptance>
<description>Inspect automation service to ensure it is up and operational</description>
<releaseNotes></releaseNotes>
<copyright>$copy$</copyright>
<tags>verifier-observation-plugin automation</tags>
<dependencies>
<group targetFramework="net462" />
</dependencies>
<references>
<group targetFramework="net462">
<reference file="Verifier.Observations.DevOps.Automation.dll" />
</group>
</references>
</metadata>
<files>
<file src="bin\*\net462\*.dll" target="lib\net462" />
<file src="bin\*\net462\*.pdb" target="lib\net462" />
</files>
</package>

Answer 1

You can control where assemblies are deployed in the nuget package using an item in an itemgroup, similar to this:
<ItemGroup>
<None Include="!!path_to_assembly!!">
<PackagePath>lib\net462</PackagePath>
<Pack>true</Pack>
<Visible>false</Visible>
</None>
</ItemGroup>
That should include the specified assembly in the package.

Answer 2

You can add the following target to your .csproj:
<Project Sdk="Microsoft.NET.Sdk">
<PropertyGroup>
<TargetFrameworks>netstandard2.0;net47</TargetFrameworks>
<TargetsForTfmSpecificBuildOutput>$(TargetsForTfmSpecificBuildOutput);CopyProjectReferencesToPackage</TargetsForTfmSpecificBuildOutput>
</PropertyGroup>
<ItemGroup>
<ProjectReference Include="..\ClassLibrary2\ClassLibrary2.csproj" PrivateAssets="all" />
<ProjectReference Include="..\ClassLibrary3\ClassLibrary3.csproj" Condition="'$(TargetFramework)' == 'net47'" PrivateAssets="all" />
</ItemGroup>
<Target Name="CopyProjectReferencesToPackage" DependsOnTargets="ResolveReferences">
<ItemGroup>
<BuildOutputInPackage Include="#(ReferenceCopyLocalPaths->WithMetadataValue('ReferenceSourceTarget', 'ProjectReference'))" />
</ItemGroup>
</Target>
</Project>
Source 1
Source 2
Reference: Advanced extension points to create customized package