New linter for detecting non-exported unused objects in a package

[IndrajeetPatil]

These are typically left behind once the codebase has been refactored (e.g., r-lib/styler#989, r-lib/devtools#2453, r-lib/downlit#129, etc.), and form technical debt.

It would be nice if they produce a lint so that the developers can immediately see which internal objects are no longer needed and can be safely removed.

Possible name: unused_internal_object()

[AshesITR]

This will only be possible if we implement a way to lint packages as a whole. The file-based approach would fire way too many false positives since internal utilities are usually defined in a different file than they are used in.

[IndrajeetPatil]

Yes, definitely!

I think this linter makes sense only in the context of lint_package(), but not otherwise.

[AshesITR]

I can also see this being useful in general lint_dir() (e.g. if you have some Rmds and a util.R file with helpers).