Limit default apphost feature to netcoreapp3.0

[peterhuene]

Steps to reproduce

Using a 3.0 SDK:

1. dotnet new console.
2. Change the TargetFramework property to netcoreapp2.1.
3. Build.

Expected behavior

There should be no default apphost present.

Actual behavior

A default apphost (.exe) is created.

There are two reasons why this is undesirable:

1. The default apphost feature was implemented in such a way that impacts restore; namely that a restore for the current SDK's platform occurs so the apphost package appears in the assets graph. This has a performance impact on dotnet commands that do a restore and unnecessarily pollutes the assets graph. The hope was that we would support "download only" packages that don't affect the restore operation and assets graph. This feature is coming online to support framework and targeting packs, but is limited to netcoreapp3.0+.

2. Users that upgrade their SDK, even for major SDK releases, should reasonably expect no new build outputs when their target framework has not changed. This is the reason we limit the "copy local dependencies on build" feature to netcoreapp3.0+.

Environment data

.NET Core SDK (reflecting any global.json):
 Version:   3.0.100-preview-009812
 Commit:    e3abf6e935

Runtime Environment:
 OS Name:     Windows
 OS Version:  10.0.17763
 OS Platform: Windows
 RID:         win10-x64
 Base Path:   C:\Program Files\dotnet\sdk\3.0.100-preview-009812\

Host (useful for support):
  Version: 3.0.0-preview-27122-01
  Commit:  00c5c8bc40

.NET Core SDKs installed:
  2.1.402 [C:\Program Files\dotnet\sdk]
  2.1.403 [C:\Program Files\dotnet\sdk]
  2.1.500 [C:\Program Files\dotnet\sdk]
  3.0.100-preview-009793 [C:\Program Files\dotnet\sdk]
  3.0.100-preview-009812 [C:\Program Files\dotnet\sdk]

.NET Core runtimes installed:
  Microsoft.AspNetCore.All 2.1.4 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All]
  Microsoft.AspNetCore.All 2.1.5 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All]
  Microsoft.AspNetCore.All 2.1.6 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All]
  Microsoft.AspNetCore.App 2.1.4 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 2.1.5 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 2.1.6 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 3.0.0-preview-18577-0031 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 3.0.0-preview-18579-0056 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.NETCore.App 2.1.4 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 2.1.5 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 2.1.6 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 3.0.0-preview-27122-01 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.WindowsDesktop.App 3.0.0-alpha-27122-4 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
  Microsoft.WindowsDesktop.App 3.0.0-alpha-27128-4 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]

To install additional .NET Core runtimes or SDKs:
  https://aka.ms/dotnet-download

[peterhuene]

A fix for this issue should include the removal of the "resolve default apphost from assets file" plumbing.