PackageValidation runs in no-op incremental builds

[ViktorHofer]

The PackageValidation SDK feature currently runs always, even if the package hasn't changed:

sdk/src/Tasks/Microsoft.NET.Build.Tasks/targets/Microsoft.NET.Compatibility.Common.targets

Lines 10 to 12 in 69a8180

	<Target Name="RunPackageValidation"
	DependsOnTargets="$(RunPackageValidationDependsOn)"
	AfterTargets="Pack"

This is the case because the RunPackageValidation target is hooked onto the wrong target (maybe there's one that doesn't run in no-op incremental packaging builds) and because it doesn't define any Inputs and Outputs.

When incrementally building the libraries subset of dotnet/runtime for allconfigurations (build.cmd libs -allconfigurations && build.cmd libs -allconfigurations /bl), this adds 111 seconds to the overall build.

cc @safern @joperezr @ericstj

[rainersigwald]

I suspect there is not a better target to hook, but this could be avoided with inputs + outputs. Right now it looks like the only output is MSBuild properties/items/warnings/errors, but the target or task can always touch a marker file to create a new timestamped output.

[ericstj]

Agree, inputs/outputs would be more resilient here. We also need to rerun even if the package isn't rebuilt: for example, someone changes a suppression file or changes the left-hand-side package.

[ViktorHofer]

FWIW, here's another data point that I just collected from a libraries allconfigurations build: