Proj0014: Include project references only once

A <ProjectReference> should only be defined once per package.

Non-compliant

<Project Sdk="Microsoft.NET.Sdk">

  <ItemGroup>
    <ProjectReference Include="..\EmptyNodes\EmptyNodes.csproj" />
    <ProjectReference Include=".\..\..\Projects\EmptyNodes\EmptyNodes.csproj" />
  </ItemGroup>
  
</Project>

Compliant

<Project Sdk="Microsoft.NET.Sdk">

  <ItemGroup>
    <ProjectReference Include="..\EmptyNodes\EmptyNodes.csproj" />
  </ItemGroup>
  
</Project>