Support referencing netstandard packages

[CumpsD]

Description

Right now, we don't support the netstandard things yet. If I understand it correctly, it is fine to reference these from .NET 4.6 (for example) if it matches the right netstandard.

Repro steps

• Make a console app (net462)
• Use this:

source https://www.myget.org/F/aspnetvnext/api/v3/index.json
source https://www.nuget.org/api/v2/
nuget Microsoft.AspNetCore.Server.Kestrel 1.0.0-rc2-20466

Expected behavior

These references should be present: https://github.com/CumpsD/kestrel-classic/blob/master/src/KestrelClassic.fsproj#L80-L130

Actual behavior

netstandard is not referenced.

Known workarounds

Manually update .fsproj to add references.

Related information

Seems we can map them:

• https://github.com/dotnet/corefx/blob/master/Documentation/architecture/net-platform-standard.md
• https://github.com/dotnet/corefx/blob/master/Documentation/architecture/net-platform-standard.md#mapping-the-net-platform-standard-to-platforms

Does this mean if I target net462, Paket would have to also look for netstandard15 (and lower) and netcoreapp10?