Code Quality: Improved the way packages are managed in the codebase (#16432)

Author: Lamparter

nuget.config

@@ -1,7 +1,6 @@
 <?xml version="1.0" encoding="utf-8"?>
 <configuration>
     <packageSources>
-        <add key="NuGet official package source" value="https://api.nuget.org/v3/index.json" />
-        <add key="Local Packages" value="src/Files.App/nupkgs/" />
+        <add key="Project Packages" value="src/Files.App/Assets/Libraries/" />
     </packageSources>
 </configuration>

src/Files.App/nupkgs/Microsoft.UI.winmd renamed to src/Files.App/Assets/Libraries/Microsoft.UI.winmd

@@ -95,7 +95,7 @@
     </ItemGroup>
 
     <ItemGroup>
-        <CsWinRTInputs Include="nupkgs\Microsoft.UI.winmd" />
+        <CsWinRTInputs Include="Assets\Libraries\Microsoft.UI.winmd" />
         <CsWinRTInputs Include="..\Files.App.Server\bin\$(Platform)\$(Configuration)\$(TargetFramework)\win-$(Platform)\Files.App.Server.winmd" />
         <None Include="..\Files.App.Server\bin\$(Platform)\$(Configuration)\$(TargetFramework)\win-$(Platform)\Files.App.Server.winmd">
           <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>