NuGet Package should reference FSharp.Core package

[matthid]

Currently the NuGet Package references
Microsoft.FSharp.Core.netcore (>= 1.0.0-alpha-161205) for netcore, but it should use https://www.nuget.org/packages/FSharp.Core/ to be consistent across the ecosystem...

Repro steps

In FAKE vNext I use a complete paket group to compile the FAKE script. Therefore I get (when FSharp.Compiler.Service is a dependency of the group, in addition to FSharp.Core):

error FS0215: Multiple references to 'FSharp.Core.dll' are not permitted

Provide the steps required to reproduce the problem

1. Clone my FAKE coreclr branch
2. ./build.sh
3. run the use external paket.dependencies integration test by hand

(let me know if someone actually wants to do it and needs help)

Known workarounds

Maybe I should manually blacklist Microsoft.FSharp.Core.netcore in Fake.vnext.

[dsyme]

@matthid Could you send a PR for this? thanks

[matthid]

@dsyme done