Verify that version downgrades don't happen with the PVP flow.

[MichaelSimons]

It is possible the the PVP flow can cause silent downgrades. This is typically undesirable and should ideally cause a build error.

Example scenario

[mmitche]

We should be careful that any PVP flow downgrade errors don't affect previously source-built packages (or at least there is a way to work around this). I think you'll hit issues if you have a PSB provided dependency which is upgraded on backflow (maybe a toolset dep?)

• Previously source-built is the 10.0.0 runtime and 10.0.100 SDK
• In building 10.0.1, a component repo takes a backflow which updates something to 10.0.1.
• In SB mode, this dependency is provided by PSB.

[ViktorHofer]

The intent here is to remove differences around dependency versions between source-build and msft build. Currently, PvP downgrades versions to match live artifacts / PSB artifacts without raising any warnings or errors. In the example linked above Newtonsoft.Json/13.0.4 will be used by the MSFT build but Newtonsoft.Json/13.0.3 by source-build.