Proj0218: Symbol package format snupkg requires debug type portable

When creating symbols .snupkg package, having <DebugType> set to full or pdbonly results in an invalid .snupkg (or at least one that the official nuget symbol server can’t process). Meanwhile, <DebugType> value embedded results in the symbols being included in the regular binaries, defeating the purpose of the separate .snupkg file.

Non-compliant

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

  <PropertyGroup>
    <TargetFramework>net8.0</TargetFramework>
    <SymbolPackageFormat>snupkg</SymbolPackageFormat>
    <DebugType>embedded</DebugType>
  </PropertyGroup>

</Project>

Compliant

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

  <PropertyGroup>
    <TargetFramework>net8.0</TargetFramework>
    <SymbolPackageFormat>snupkg</SymbolPackageFormat>
    <DebugType>portable</DebugType>
  </PropertyGroup>

</Project>