Reduce size of the nuget fallback folder

[natemcmaster]

The NuGet fallback folder for 2.1 is a whopping 1.01 GB once expanded on disk. This includes lots of packages that should be unnecessary to execute a .NET Core 2.1 application, but are still required to restore a .NET Core 2.1 application.

This includes:

• multiple copies of the same package (Newtonsoft.Json 9, 10, and 11)
• multiple versions of NETStandard.Library (1.6.0, 1.6.1, and 2.0.3)
• a bunch of runtime.native.* specific packages. In theory, these should only be consumed when users are producing a self-contained app, but this is out of scope for the offline cache
• 670 MB of .xml docs
• 147 MB of .nupkg files (why are they needed -- the contents of the .nupkg have already been unzipped)

Result:

• bloated Docker images
• longer first-run experience required to expand all these files
• wasted disk space on CI machines (xml docs are typically only used by IDEs)

More details

A peek inside. Here are the packages that consume the most disk space:

Also, here is a breakdown of the files inside:

Here are all the runtime.* specific nupkgs inside

[natemcmaster]

cc @DamianEdwards @JunTaoLuo @weshaggard - you're already aware of this issue, but figured we could use this to keep track of ongoing investigation on this issue.

[weshaggard]

cc @Petermarcu

[natemcmaster]

Related issues

AzureAD/azure-activedirectory-identitymodel-extensions-for-dotnet#925
dotnet/roslyn#26299
NuGet/Home#6516

[aspnet-hello]

This issue was moved to dotnet/aspnetcore#3650