Re-design how to get "type: platform" information in MSBuild

[eerhardt]

In order to tell if an application is 'portable' or not, we need to get whether a dependency is marked as "type: platform", and which dependency it is.

However, that information is not available in the project.lock.json file. See NuGet/Home#2695.

Once NuGet provides that info in the project.lock.json file, we should be able to remove the TODOs in our MSBuild Tasks that need this information.

[eerhardt]

Instead of relying on NuGet's "type: platform", the new idea is to use an MSBuild property to signify which dependency is to be considered the "platform" dependency.

There will be an MSBuild property we'll call MicrosoftNETCorePlatformPackage for now (named TBD). This property will have the name of the dependency to treat as the "platform" dependency. The .NET Core tools will look at this property when it needs to get the name of the "platform" dependency for use in determining what dependencies to trim out in portable apps.

If a NuGet package wants to be considered a “platform” package, it can create a “build/TFM/MyPackageId.props” file in its nupkg that looks something like the following:

<Project xmlns="http://schemas.microsoft.com/developer/msbuild/2003">

  <PropertyGroup>
    <MicrosoftNETCorePlatformPackage>MyPackageId</MicrosoftNETCorePlatformPackage>
  </PropertyGroup>

</Project>

This way, users just need to reference the package, and it will automatically be used as the platform dependency.

If a user wants to manually set which package should be the platform dependency, they can set this property directly in their project.