Add Microsoft.Extensions.ObjectPool 6.0.0

[mthalman]

Adds Microsoft.Extensions.ObjectPool 6.0.0 due to prebuilt being introduced for this package from https://github.com/dotnet/razor. This looks to have been caused by dotnet/razor#8289.

[MichaelSimons]

FYI, From https://github.com/dotnet/source-build-reference-packages#reference

After generating new reference packages, all new projects must be referenced as a DependencyPackageProjects. These must be defined in dependency order.

[mthalman]

FYI, From https://github.com/dotnet/source-build-reference-packages#reference

After generating new reference packages, all new projects must be referenced as a DependencyPackageProjects. These must be defined in dependency order.

This is what confuses me. Why is only one listed when there are a whole bunch of other packages in src?

[MichaelSimons]

FYI, From https://github.com/dotnet/source-build-reference-packages#reference

After generating new reference packages, all new projects must be referenced as a DependencyPackageProjects. These must be defined in dependency order.

Only new projects are added here until the previous source-build-artifacts get updated in which they are removed. See for details: https://github.com/dotnet/source-build-reference-packages/blob/main/eng/Build.props#L11. This all related to the problem of project references not being utilized in the projects.