NuGet: Store source packages

Even though NuGet is a binary package manager (with links to external VCS), there seems to be some source packages, such as: https://www.nuget.org/packages/LightInject.Source

This needs to be investigated, and may deserve its own loader.

Migrated from T4624 (view on Phabricator)

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information