Namespace linter

[renkun-ken]

#547 implements a missing_package_linter() which checks if all packages loaded by library(), require(), loadNamespace(), and requireNamespace() are installed.

Apart from that, we could also check the usage of ns::obj and ns:::obj as they also used to access objects in a namespace (provided by package).

We could implement a separate linter (as suggested at #547 (comment)) to check if

1. The package ns in ns::obj and ns:::obj are installed.
2. The obj in ns::obj is exported from namespace ns.
3. The obj in ns:::obj exists in namespace ns.

While checking 1 does not load any new package, checking 2 and 3 might load the package being checked. I'm not sure if we allow loading extra packages from user code?

[AshesITR]

We already run user code for some linters:

https://github.com/jimhester/lintr/blob/1a618f3f7df4889fe39eff5e0ce4e5c0fc6eadfd/R/object_usage_linter.R#L1-L4

Maybe make these checks optional with a parameter and document that it may load packages if the parameter (check_exports = TRUE?) is set?