Add common external package version variables

[BrennanConroy]

@mikaelm12 @pranavkm @natemcmaster

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[natemcmaster]

What will be the corresponding change for something that consumes the Internal.AspNetCore.Sdk project, like aspnet/Common?

[pranavkm]

@natemcmaster - we're going to update individual repos to include this value. I guess we can do that once this package builds.

[mikaelm12]

😄

[natemcmaster]

I don't think that'll work. The package isn't installed when you run restore for the first time.

[natemcmaster]

Recommend either creating Internal.AspNetCore.Test.Sdk which depends on the xunit version we need, or waiting until dotnet/msbuild#1493 is ready