Directory.Build.targets is not imported for multi-targeting projects where TargetFramework is not defined

[AArnott]

When a "Microsoft.NET.Sdk" project defines a TargetFrameworks property instead of a TargetFramework property (effectively activating the multi-targeting behavior), it no longer imports Directory.Build.targets in the top-level evaluation.

When a TargetFramework is set, it imports both Directory.Build.props and Directory.Build.targets.
But when only TargetFrameworks is defined, it only imports Directory.Build.props.

This distinction is very subtle and unexpected. It took me quite some time to diagnose a build issue before I figured this out. I trust this was an unintentional oversight that can be fixed.

Repro project:
dirprops.zip

When building the Restore target, I see this:

msbuild /t:restore
  Restoring packages for C:\temp\dirprops\lib\lib.csproj...
  Lock file has not changed. Skipping lock file write. Path: C:\temp\dirprops\lib\obj\project.assets.json
  Restore completed in 378.45 ms for C:\temp\dirprops\lib\lib.csproj.

  NuGet Config files used:
      C:\Users\andarno\AppData\Roaming\NuGet\NuGet.Config
      C:\Program Files (x86)\NuGet\Config\Microsoft.VisualStudio.Offline.config

  Feeds used:
      https://api.nuget.org/v3/index.json
      C:\Program Files (x86)\Microsoft SDKs\NuGetPackages\
  AddInProps is executing...

But when I define TargetFramework I see both targets executing:

msbuild /t:restore /p:targetframework=netstandard1.3
  Restoring packages for C:\temp\dirprops\lib\lib.csproj...
  Generating MSBuild file C:\temp\dirprops\lib\obj\lib.csproj.nuget.g.props.
  Generating MSBuild file C:\temp\dirprops\lib\obj\lib.csproj.nuget.g.targets.
  Writing lock file to disk. Path: C:\temp\dirprops\lib\obj\project.assets.json
  Restore completed in 764.25 ms for C:\temp\dirprops\lib\lib.csproj.

  NuGet Config files used:
      C:\Users\andarno\AppData\Roaming\NuGet\NuGet.Config
      C:\Program Files (x86)\NuGet\Config\Microsoft.VisualStudio.Offline.config

  Feeds used:
      https://api.nuget.org/v3/index.json
      C:\Program Files (x86)\Microsoft SDKs\NuGetPackages\
  AddInProps is executing...
  AddInTargets is executing...

[rainersigwald]

Duplicate of #1721, fixed for the 15.3 release.