Use fewer large item groups (#30537)
Use fewer large item groups (#30537)

- do not dupe the `@(ProjectReferenceProvider)` item group
- not a big performance win but is a bit more readable
  - before: https://dev.azure.com/dnceng/public/_build/results?buildId=1009384
    - average job took 21m 38s; average artifact size 144.15MB
  - after:  https://dev.azure.com/dnceng/public/_build/results?buildId=1009382
    - average job took 21m 2s;  average artifact size 138.65MB
aspnet/AspNetCore,aspnet/AspNetCore,aspnet/AspNetCore,aspnet/AspNetCore,aspnet/AspNetCore,aspnet/AspNetCore,aspnet/AspNetCore,aspnet/AspNetCore,aspnet/AspNetCore