Require all projects participating in the build to import Common targets

Currently some projects (e.g. pkgproj in CoreFX) do not do so and thus features provided by Common targets such as `Directory.Build.props|targets` and `CustomBeforeMicrosoftCommonTargets` do not work and require workarounds in Arcade SDK.

Start by examining the pkgproj projects referenced in CoreFX and how they import Common targets, including Directory.Build.props|targets and CustomBeforeMicrosoftCommonTargets. Compare this with the Arcade SDK workaround; done means every project participating in the build imports the Common targets so those features work without project-specific workarounds.

Written by the indexing model from the issue text.