chore: consolidate MSBuild configuration barriers

- Moved Directory.Build.* barriers from nuke to root folder to cover all projects.
- Added a Directory.Packages.props barrier to stop central package management.
- Removed obsolete 'ManagePackageVersionsCentrally' properties.

Directory.Packages.props

@@ -0,0 +1,8 @@
+<?xml version="1.0" encoding="utf-8"?>
+<Project ToolsVersion="4.0" DefaultTargets="Build" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
+
+  <!-- This file prevents unintended imports of unrelated MSBuild files -->
+  <!-- Uncomment to include parent Directory.Packages.props file -->
+  <!--<Import Project="$([MSBuild]::GetPathOfFileAbove('Directory.Packages.props', '$(MSBuildThisFileDirectory)../'))" />-->
+
+</Project>