Multi-target build task to net4x/netcoreapp20

[nguerrera]

.NET Standard msbuild tasks are problematic and just about everyone has converged on multi-targeting instead (including SDK and core nuget tasks). dotnet/msbuild#1309 is one example of the problems that can be hit. There are workarounds, but it is fragile.

Recently, the build task added a dependency on Pipes.AccessControl that is not part of the shared framework, so it needed to be included in the package. To do this, the netstandard1.3 was published dropping a bunch of other unnecessary dependencies next to the build tasks and breaking crossgen upon insertion in to the CLI.

Publishing a netstandard library is basically a broken scenario. See dotnet/sdk#1855 (comment), which goes on to discuss warnings/errors for this case. When you publish, you should really use a "runnable" TFM like net46 or netcoreapp2.0.

For now, CLI is deleting the extra assemblies to unblock the insertion, but this isn't sustainable because it's implemented with fragile wild-cards ("System.*", but not Pipes.AccessControl) that are subject to change at any time.

Assigning to @jaredpar as he asked me to do so offline.

[jaredpar]

This is done.