Extend namespace_linter() for using '::' for functions already imported in the NAMESPACE

[MichaelChirico]

#' @importFrom pkg foo
pkg::foo()

The latter usage of foo() could be linted as pkg:: is not needed.

Such a linter could help backsliding after clean-ups like #2031

[AshesITR]

Add it to the namespace linter?

I think it fits well there, since it's examining the NAMESPACE of the linted package.