.NET 包对不同框架的兼容性

原issue:https://github.com/NuGet/Home/issues/9604

解决方案：

　　默认兼容性提供程序：https ://github.com/NuGet/NuGet.Client/blob/6fd451a5099c6f3d0b17035915d6171a744c4c6b/src/NuGet.Core/NuGet.Frameworks/DefaultCompatibilityProvider.cs

　　IsCompatible方法：https ://github.com/NuGet/NuGet.Client/blob/6fd451a5099c6f3d0b17035915d6171a744c4c6b/src/NuGet.Core/NuGet.Frameworks/def/IFrameworkCompatibilityProvider.cs#L20

其他：https://stackoverflow.com/questions/39212007/check-if-package-is-compatible-with-net-core