Handle VS 2017 projects for .NET classic

[skolima]

Test uses a sample from 4.6.1 VS2017 project format, with a namespace and slightly different version storage (node instead of attribute).

[AnthonySteele]

There seem to be 2 things there -
An xml namespace at the root node, and that the package's Version can be an element not an attr on the PackageReference?

I see a doc saying "all metadata to be specified as a attribute or child element":
https://github.com/NuGet/Home/wiki/PackageReference-Specification
So that's correct

[skolima]

Turns out the updater needs to be changed slightly as well, otherwise NuKeeper will put two versions of the dependency in.

[skolima]

Relevant NuGet/Home#4358 https://github.com/dotnet/cli/issues/5425

[skolima]

Workaround used for dotnet add package not removing old version every time: run dotnet remove package first.