Make CpuMath not depending on ML.Core again

[wschin]

Fixes #1688.

CpuMath project now includes Contract.cs in ML.Core. The file Contract.cs contains some if-else macros for distinguishing the part for general ML.NET (namespace: Microsoft.ML) from the part for CpuMath(namespace: Microsoft.ML.Runtime.Internal.CpuMath.Core). The BestFriendAttribute in ML.Core (namespace: Microsoft.ML) and CpuMath (namespace: Microsoft.ML.Runtime.Internal.CpuMath.Core) follow the same pattern for the same reason. BestFriendAnalyzer is also modified to support the two variants of BestFriend attributes.

Besides, we move PublicKey into another file, PublicKey.cs, to avoid including AssemblyInfo.cs twice in CpuMath project.

[eerhardt]

// Licensed to the .NET Foundation under one or more agreements.

(nit) I think it makes more sense for this file to live outside of the Properties folder, and next to the BestFriendAttribute.cs file. They are in the same "namespace". #Resolved

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Refers to: src/Microsoft.ML.Core/Properties/PublicKey.cs:1 in 7f374c6. [](commit_id = 7f374c6, deletion_comment = False)

[eerhardt]

Just the question about if using #if PRIVATE_CONTRACTS as the name still makes sense, and a minor question about the old namespace.

Other than those 2 things, this looks good.

[Ivanidzo4ka]