Dual configurations (specified defaults AND project-level override)

[zachrybaker]

It would be really helpful to be able to have dual configurations, where the "global install" (i.e. the .nuget AppData location) had the "default machine-wide" settings, which were read in, and yet if a project-level config file existed, those were allowed to override for whatever settings were found there.

The scenario we're after is having a default for a developer's machine, or a build server, which specifies things like the "normal" behaviors as well how to key the vulnerability APIs, and those settings get tweaked as needed for a project that has a very minimal set of settings (that only came into play b/c they were needed).

For instance perhaps a solution with 90 projects is going to have rate-limiting issues, but most won't. Or perhaps we ignore certain packages but only on certain projects. Etc.