Use empty project collection inside Project.FromFile()

[MattGal]

To double check:

• The right tests are in and and the right validation has happened. Guidance: https://github.com/dotnet/arcade/tree/main/Documentation/Validation

Testing ongoing; my first attempt used a Warning() call and I'm having trouble treating warnings as warnings in the ASP build
When this publishes I can try it: https://dnceng.visualstudio.com/internal/_build/results?buildId=1926170&view=results

[MattGal]

Tested by @lbussell , (I had some challenges with local sourcebuild setup), I believe this fixes the issue seen in some source build scenarios.

[lbussell]

I ran darc-update with these changes and then ran the source-build repo leg locally and it appeared to build cleanly.