title: C# Incremental Source Generator Not Triggering build when generated files are modified
body: <p>I have 2 projects, IE: projX.csproj and projY.csproj.</p>
  <p>My src generator is referenced by projX, but the code generated is based on classes on projY.</p>
  <p>When projY classes change, the generated files should change (and they do), but because no change occurs on projX, the changes are not picked up and no build occurs on projX.
  Routes that I'm considering but can't find an answer for:</p>
  <ol>
  <li>How do I make projX recognize that the generated files have changed?</li>
  <li>Otherwise, how can I force projX to build when projY changes?</li>
  </ol>
  <p>Thank you.</p>