Walk TFMs in parallel when collecting pack outputs

[baronfel]

NuGet Product(s) Affected

dotnet.exe

Current Behavior

Today, the _WalkEachTargetPerFramework target uses a large number of MSBuild Tasks in single-item batches to collect outputs to put into the final package. Each Task invocation incurs some overhead, and each Task invocation must be built sequentially. This results in relatively long and straight units of work. This looks like this:

Desired Behavior

_WalkEachTargetPerFramework should issue batches of parallel work to the MSBuild Task invocations so that the engine can schedule the parallel work items more efficiently. This should result in increased performance with no change to functionality. That would look like this:

Additional Context

No response