PackageReference items that come from package's targets files should not be included in the set we push to NuGet

Command-line restore will not import package reference's target files during restore phase (via ExcludeRestorePackageImports), but due to VS's single evaluation model we'll include any PackageReference items that are present when we push packages to NuGet. We should deliberately filter package references out that come from package targets to have the same behavior as command-line restore.