NuGet updater doesn't handle `<PackageReference>` nodes in `.props`/`.targets` files

A `.props`/`.targets` file containing a `` node (e.g., `Directory.Build.targets`) is not updated when trying to process the relevant `.csproj` which doesn't have the node, but imports the `.props`/`.targets`.

Locate the NuGet updater entry point and reproduce the case using a .csproj that imports a Directory.Build.targets or .props file containing a PackageReference node. Add regression coverage for the imported node, then verify that the relevant dependency is updated when the project itself has no PackageReference.

Written by the indexing model from the issue text.