Fix ability for individual projects to enable or disable Central Package Management

[jeffkl]

Bug

Fixes: NuGet/Home#13560

Regression? Last working version:

Description

Pull request #5572 enabled or disabled Central Package Management (CPM) if a file named Directory.Packages.props exists. It also changed when Directory.Packages.props was imported. Before, Directory.Packages.props was always imported, whether CPM was enabled or not. in #5572 this changed to only import Directory.Packages.props was imported. This breaks users who:

1. Disable CPM in Directory.Build.props
2. Enable CPM in a particular project

This is because Directory.Packages.props isn't imported anymore and its impossible for a project to enable it.

This change brings back the old behavior of always importing Directory.Packages.props even if CPM is disabled. The ManagePackageVersionsCentrally property still controls whether or not CPM itself is enabled, allowing users to enable CPM for any particular project. In the case where a users wants to disable the import of Directory.Packages.props, they can still set the MSBuild property ImportDirectoryPackagesProps to false.

PR Checklist

• PR has a meaningful title

• PR has a linked issue.

• Described changes

• Tests

  • Automated tests added
  • OR
  • Test exception
  • OR
  • N/A

• Documentation

  • Documentation PR or issue filled
  • OR
  • N/A