Automatically determine import context for Directory.Build.props

[mhutch]

When editing a Directory.Build.props, it should be edited in the context of being imported by one of the project files in the directory cone (or ideally, with context from all of them).

[zdavidsen]

I'd love to see broader general support for this. I've built a very custom template project that imports a supplementary msbuild file (~200 lines), and the editor doesn't have any idea of the context because the supplementary file doesn't actually import its own dependencies.