make the workload resolver only light up for the specific entrypoint SDKs we control

[baronfel]

This is part of #26009

This makes NuGet/Home#13471 easier to diagnose because it removes the workload resolver from the list of resolvers - you can see the before and after here:

before:

D:\temp\test01\test01.csproj : error : Could not resolve SDK "MSTest.Sdk". Exactly one of the probing messages below indicates why we could not resolve the SDK. Investigate and resolve that message to correctly specify the SDK.
D:\temp\test01\test01.csproj : error :   SDK resolver "Microsoft.DotNet.MSBuildWorkloadSdkResolver" returned null.
D:\temp\test01\test01.csproj : error :   Unable to resolve 'MSTest.Sdk (= 3.3.1)' for '.NETStandard,Version=v0.0'. PackageSourceMapping is enabled, the following source(s) were not considered: Microsoft Visual Studio Offline Packages, NuGet official package source.
D:\temp\test01\test01.csproj : error MSB4236: The SDK 'MSTest.Sdk' specified could not be found.

after:

E:\Code\Scratch\test-app\test-app.csproj : error : Could not resolve SDK "MSTest.Sdk". Exactly one of the probing messages below indicates why we could not resolve the SDK. Investigate and resolve that message to correctly specify the SDK.
E:\Code\Scratch\test-app\test-app.csproj : error :   Unable to resolve 'MSTest.Sdk (= 3.3.1)' for '.NETStandard,Version=v0.0'. PackageSourceMapping is enabled, the following source(s) were not considered: nuget.
E:\Code\Scratch\test-app\test-app.csproj : error MSB4236: The SDK 'MSTest.Sdk/3.3.1' specified could not be found.

We save one line here, and with additional work on the MSBuild side we may be able to reduce the MSBuild resolver wrapper noise in this message.

[dsplaisted]

The workload resolver also resolves imports from workload packs, using the pack ID. So the pattern as is is insufficient. It needs to cover things like Microsoft.Android.Sdk.net9, Microsoft.NETCore.App.Runtime.AOT.Cross.net6.android-x86, and any other workload pack that has targets in it (normally these are imported via the WorkloadManifest.targets files).