object_usage_linter() should skip objects transitively attached by Depends

[MichaelChirico]

library(bit64) # NB: as of now, bit64 Depends on bit, but that will likely change in the future

foo <- function(x) {
  bit()
}

Linting this will lint on bit:

warning: [object_usage_linter] no visible global function definition for 'bit'
  bit()
  ^~~

But because library(bit64) necessarily attaches {bit}, I think it shouldn't.

[MichaelChirico]

A note for testing: as of today, {jsonlite} Depends on {methods}, so possibly we could test this in a sub-process where {methods} is not attached. None of our other direct dependencies use Depends at all.

Otherwise, we'd want to add a Suggests dependency on {bit64}.

[MichaelChirico]

Elsewhere I've seen as.environment("package:bit64")$.Depends used to get package dependencies, but this environment is only populated if {bit64} is attached. Here again #912 would help. Barring that, we can do what attachNamespace() does, which is to get Depends from DESCRIPTION.