Better trimming support + trimming fix

[mmitche]

• Allow for trimming to be set in SourceBuild.props. This enables use in repo SB legs. Includes a default set which is all netcoreapps + netstandards.
• Disable trimming for top level projects in Arcade, so that we don't accidentally skip things like publishing.

To double check:

• The right tests are in and and the right validation has happened. Guidance: https://github.com/dotnet/arcade/tree/main/Documentation/Validation

[mmitche]

I made some tweaks to this, so I'm retesting locally.

[mmitche]

One question I had was why Tools.proj looks like this:

https://github.com/dotnet/arcade/blob/main/src/Microsoft.DotNet.Arcade.Sdk/tools/Tools.proj#L6-L13

I didn't disable trimming in this project yet. I think it might mean that the NuGetRepack pack will not get restored. Anyways, my main question is why this project explicitly sets a bunch of TFM properties.

[NikolaMilosavljevic]

Just curious, where are these properties consumed? NoTargetFrameworkFiltering and DotNetTargetFrameworkFilter

[mmitche]

https://github.com/dotnet/arcade/blob/main/src/Microsoft.DotNet.Arcade.Sdk/tools/TargetFrameworkFilters.BeforeCommonTargets.targets#L4-L18

[NikolaMilosavljevic]

@MichaelSimons

[mmitche]

@NikolaMilosavljevic Slight tweak here to the default TFM string (need re-approval). This did work in source build with it enabled to sdk +installer.