Move NuGet xplat to Microsoft.NetCore.App and netcoreapp1.0

[TheRealPiotrP]

CLI + Extensions are stabilizing on TFM netcoreapp1.0 and dependency package Microsoft.NetCore.App for RC2. The same tfm and dependencies should be reflected in NuGet xplat client.

[yishaigalatzer]

@piotrpMSFT we took a change, but I assume this is not going to meet ask mode, so taking for RTM.

@pakrym this is what you just committed right?

[TheRealPiotrP]

Ok. We're taking a bunch of changes like this for RC2 at the moment. If needed, we'll leave imports for nuget.

[yishaigalatzer]

Feel free to make an ask for RC2, we could look at merging into 3.5Beta early next week. We are all heads down on shipping 3.4.2 tomorrow

[pakrym]

Fixed by NuGet/NuGet.Client#472