Pin tool and package versions to make builds more repeatable

[natemcmaster]

Part of aspnet/Universe#575 and aspnet/KoreBuild#239

This removes floating versions of dependencies. This includes the korebuild version and versions used by PackageReference's.

Changes:

• build.cmd /t:Pin is no longer required to use Mvc.sln
• (script generated) replace PackageLineup with MSBuild variables for dependencies versions

[natemcmaster]

cc @javiercn - as a part of this change, we're also verifying that packages produced by a repo have consistent versions. I noticed that Microsoft.AspNetCore.Mvc.Testing is 2.1.0-alpha1, but the rest of the repo is 2.1.0-preview1. What was the reason for this change? #6521 I didn't see any discussion or sign-off.

[javiercn]

@natemcmaster It happened offline. We wanted to have an alpha that people could use with 2.0. It is safe at this point to make it 2.1.0-preview1 like the rest of the packages

[natemcmaster]

Ok, thanks for the clarification. I'll update that in this PR too so we don't get build warnings.