Restructure product DLLs maintaining compatibility with old naming scheme

In order to more effectively collect crash reports and differentiate them from .NET Framework reports, we want to ensure that all binaries are named differently than they are in .NET Framework. This needs to be done by Preview 7 as this is, essentially, a binary composition and API change.

This was already done for the native assemblies (e.g. penimc_**cor3**), we should use the same suffix (**_cor3**).

There are several steps to follow here:
- [ ] Rename outputs in all projects (open and internal)
- [ ] Update ProjectReferences in all projects (open/internal/test)
- [ ] Update targets related to replacing ProjectReferences according to open/internal split
- [ ] Update any System.Xaml/PBT code that might rely on specific names
- [ ] Ensure PBT works in our tests (manual and otherwise)
- [ ] Full test runs
- [ ] Fix any test issues arising from the rename
- [ ] Create type forwarders for all assemblies using GenAPI (use different project names to libs)
* These will be static and represent a snapshot in time of the API, specifically compatible with .NET 4.8
- [ ] Update ApiCompat to ensure these forwarders are always compatible with .NET 4.8
- [ ] Inform upstream consumers of the changes to the binaries (see [here ](https://github.com/dotnet/core-setup/issues/6694)for an example).

This list will update as things progress.