Create framework assembly list file in .NET Core targeting pack

[dagood]

This adds data/FrameworkList.xml to the targeting pack with contents like this:

<FileList Name=".NET Core 3.0" TargetFrameworkIdentifier=".NETCoreApp" TargetFrameworkVersion="3.0" FrameworkName="Microsoft.NETCore.App">
  <File AssemblyName="Microsoft.CSharp" PublicKeyToken="b03f5f7f11d50a3a" AssemblyVersion="4.0.0.0" FileVersion="4.7.19.11304" />
  <File AssemblyName="Microsoft.VisualBasic.Core" PublicKeyToken="b03f5f7f11d50a3a" AssemblyVersion="10.0.4.0" FileVersion="4.7.19.11304" />
  <File AssemblyName="Microsoft.VisualBasic" PublicKeyToken="b03f5f7f11d50a3a" AssemblyVersion="10.0.0.0" FileVersion="14.7.3060.0" />
  <File AssemblyName="Microsoft.Win32.Primitives" PublicKeyToken="b03f5f7f11d50a3a" AssemblyVersion="4.1.1.0" FileVersion="4.7.19.11304" />
  <File AssemblyName="mscorlib" PublicKeyToken="b77a5c561934e089" AssemblyVersion="4.0.0.0" FileVersion="4.7.3060.0" />
  <File AssemblyName="netstandard" PublicKeyToken="cc7b13ffcd2ddd51" AssemblyVersion="2.1.0.0" FileVersion="1.1.19.7403" />
...

This is for https://github.com/dotnet/core-setup/issues/5074.

It might make more sense to name it Microsoft.NETCore.App.Ref.FrameworkList.txt, like the platform manifest--just went with what I first heard since I don't think there's a spec for this yet.

@dsplaisted @nguerrera

[dsplaisted]

The path/name for the file look good to me. I think we will eventually want the platform manifest to be data/PlatformManifest.txt.

[dagood]

No time like the present--renamed the platform manifest.

[nguerrera]

In that spirit, we're leaving ref/tfm for the actual reference assemblies as discussed yesterday...

[dagood]

Changing the package layout is as simple as removing a target that moves stuff from ref/ to data/, but it has some ripple effects so I'll work on that in another PR.