Additional information about shared frameworks to bundled versions

[dsplaisted]

Adds known framework information for the base .NET Core shared framework, and adds information about runtime packs for shared frameworks. This information will be consumed by logic in dotnet/sdk#2774.

With this PR, the known framework information looks like this:

    <KnownFrameworkReference Include="Microsoft.NETCore.App"
                              TargetFramework="netcoreapp3.0"
                              RuntimeFrameworkName="Microsoft.NETCore.App"
                              DefaultRuntimeFrameworkVersion="3.0.0-preview-27218-01"
                              LatestRuntimeFrameworkVersion="3.0.0-preview-27218-01"
                              TargetingPackName="Microsoft.NETCore.App"
                              TargetingPackVersion="3.0.0-preview-27218-01"
                              AppHostPackNamePattern="runtime.**RID**.Microsoft.NETCore.DotNetAppHost"
                              AppHostRuntimeIdentifiers="freebsd-x64;linux-arm;linux-arm64;linux-musl-x64;linux-x64;osx-x64;rhel.6-x64;tizen.4.0.0-armel;tizen.5.0.0-armel;win-arm;win-arm64;win-x64;win-x86"
                              RuntimePackNamePatterns="runtime.**RID**.Microsoft.NETCore.App;runtime.**RID**.Microsoft.NETCore.DotNetHostResolver;runtime.**RID**.Microsoft.NETCore.DotNetHostPolicy"
                              RuntimePackRuntimeIdentifiers="freebsd-x64;linux-arm;linux-arm64;linux-musl-x64;linux-x64;osx-x64;rhel.6-x64;tizen.4.0.0-armel;tizen.5.0.0-armel;win-arm;win-arm64;win-x64;win-x86"
                              />
    
    <KnownFrameworkReference Include="Microsoft.WindowsDesktop.App"
                              TargetFramework="netcoreapp3.0"
                              RuntimeFrameworkName="Microsoft.WindowsDesktop.App"
                              DefaultRuntimeFrameworkVersion="3.0.0-alpha-27218-5"
                              LatestRuntimeFrameworkVersion="3.0.0-alpha-27218-5"
                              TargetingPackName="Microsoft.WindowsDesktop.App"
                              TargetingPackVersion="3.0.0-alpha-27218-5"
                              RuntimePackNamePatterns="runtime.**RID**.Microsoft.WindowsDesktop.App"
                              RuntimePackRuntimeIdentifiers="win-x64;win-x86"
                              />

    <KnownFrameworkReference Include="Microsoft.AspNetCore.App"
                              TargetFramework="netcoreapp3.0"
                              RuntimeFrameworkName="Microsoft.AspNetCore.App"
                              DefaultRuntimeFrameworkVersion="3.0.0-preview-18579-0056"
                              LatestRuntimeFrameworkVersion="3.0.0-preview-18579-0056"
                              TargetingPackName="Microsoft.AspNetCore.App"
                              TargetingPackVersion="3.0.0-preview-18579-0056"
                              RuntimePackNamePatterns="runtime.**RID**.Microsoft.AspNetCore.App"
                              RuntimePackRuntimeIdentifiers="win-x64;win-x86;win-arm;osx-x64;linux-musl-x64;linux-x64;linux-arm;linux-arm64"
                              />

The runtime identifiers for which runtime packs and apphost packs are available are generated by parsing the runtime.json from the corresponding NuGet package (e.g. Microsoft.NETCore.App). This may change as we evolve what targeting packs look like. @ericstj @natemcmaster