Make `resource()` function public

[peterdesmet]

For developers, it would be useful if the resource() function (previously get_resource()) was a public function to access a resource (list) by its name.

[PietrH]

Is this blocked by #285?

[peterdesmet]

Is this blocked by #285?

Not necessarily. It's just a use case that came up in etn. In most cases it's beneficial to know a path is invalid.