Create targeting pack for ASP.NET Core

[dsplaisted]

For .NET Core 3.0, we expect to separate targeting packs, which contain the assets needed at build time when targeting .NET Core 3.0 (for example, reference assemblies and intellisense documentation) from "runtime packs", which will include the platform-specific runtime components for .NET Core 3. See dotnet/cli#10006, and dotnet/designs#50

We will have targeting pack for each shared framework (.NET Core, ASP.NET Core, and WindowsDesktop), as well as for .NET Standard. This issue tracks the work required for the ASP.NET Core shared framework.

The work required includes:

• A new NuGet package including the targeting pack assets. This should include the reference assemblies, intellisense documentation, and platform manifest file. The current straw man proposal for the package ID is Microsoft.NETCore.App.Ref
• Installers for each supported installer format (msi, deb, pkg, and rpm), which lay out the targeting pack files under <DOTNET_ROOT>/packs/<PackageId>/version. These installers will be bundled or depended on by the corresponding .NET Core SDK installers.

Some of the details (such as package layout, names, etc) are still under design and may change slightly.

[natemcmaster]

Until we have verified the details of the new design, we are planning to implement this side-by-side with our existing packages. Here is our current plan:

• Create a new NuGet package Microsoft.AspNetCore.App.Ref which contains reference assemblies, docxml, and the platform manifest file
• Create zip, tar, MSI, Debian, and RPM installers which place the contents of targeting pack into $DOTNET_ROOT/packs/Microsoft.AspNetCore.App.Ref/$version
• Create new NuGet packages named Microsoft.AspNetCore.App.Runtime.$rid.
  - [x] Create zip, tar, MSI, Debian, and RPM installers which place the contents of runtime pack into $DOTNET_ROOT/packs/Microsoft.AspNetCore.App.Runtime.$rid/$version not necessary.

Details TBD:

• Layout of the contents inside the NuGet packages.
• The packageType to be used.
• How do we specify package conflict overrides?
• How do we specify platform manifests for updated runtimes? Current design only calls for these to be in the targeting pack.
• Is this going to be ready to release in Preview 3?

cc @nguerrera @pakrym