Describe the bug

When trying to install Microsoft.AspNetCore.Identity.EntityFrameworkCore 3.0.0-preview8.19405.7 in a Class Library (netstandard 2.1) it says it only supports netcoreapp3.0

Note: In SDK 2.2, Microsoft.AspNetCore.Identity.EntityFrameworkCore supports netstandard 2.0

To Reproduce

Steps to reproduce the behavior:

1. Using this version of ASP.NET Core 'SDK 3.0.100-preview8-013656'
2. Create a .NET Core Class Library (netstandard 2.1)
3. dotnet add package Microsoft.AspNetCore.Identity.EntityFrameworkCore --version 3.0.0-preview8.19405.7
4. See error: 'error: Package Microsoft.AspNetCore.Identity.EntityFrameworkCore 3.0.0-preview8.19405.7 is not compatible with netstandard2.1 (.NETStandard,Version=v2.1) Package Microsoft.AspNetCore.Identity.EntityFrameworkCore 3.0.0-preview8.19405.7 supports: netcoreapp3.0 (.NETCoreApp,Version=v3.0)
   error: Package 'Microsoft.AspNetCore.Identity.EntityFrameworkCore' is incompatible with 'all' frameworks in project'

Expected behavior

Microsoft.AspNetCore.Identity.EntityFrameworkCore supports netstandard 2.1