Platform dependency change detection

[mthalman]

Summary

Detecting dependency changes when they are first made to source code allows us to capture right away when such changes are being introduced. This allows a workflow which can potentially block incompatible changes (see dotnet/runtime#38755) or provide a developer experience whereby they can easily update the platform dependency data so that the dependency is appropriately tracked.

Note: This is a proposed experience for .NET 6 and not yet committed.

Goals

• .NET contributors can follow a well-defined process when they make changes to the platform dependencies.
• .NET contributors can be made aware when they've made a change that affects .NET's runtime or build-time platform dependencies.
• .NET contributors can easily update .NET's required package version for a platform dependency.
• .NET contributors can be made aware when they've added a platform dependency that cannot be satisfied by a supported OS.
• Platform dependency stakeholders are made aware of changes to the dependencies.