Detect and report package shadowing

[Enchufa2]

Initial motivation: eddelbuettel/r2u#30. It would be nice to have a mechanism to detect and report shadowed packages, i.e. packages that are available in several .libPaths(). In this first iteration, I devise:

1. An exported function, e.g. bspm::shadowed_packages(), which would work as a thin wrapper around installed.packages(). It would filter its output to detect package shadowing and would return a matrix (to follow the same convention) with at least the following columns: Package, LibPath, Version, ShadowPackage, ShadowLibPath, and ShadowVersion or similar naming. Basically, the idea is that each row is a Package that is being shadowed by ShadowPackage in a ShadowLibPath with higher priority.
2. A non-exported function that takes the output from (1) and generates one message (warning?) per row to alert the user about shadowed packages.
3. Integration of the alert messages (with shut-off switches via options). Possible places that come to mind:
   • On bspm startup.
   • On package installation, a message/warning for packages that have another one shadowing them. This would be available only for the default slower-but-safer mode, not in the binary-source mode.
   • ...?

@eddelbuettel As always, comments welcome.

[eddelbuettel]

No comment with code yet, been meaning to write one for my own use (as I believe I mentioned to your prior to eddelbuettel/r2u#30) and would (at least locally) default to sort via data.table. If and when I have something I may drop here.

The need is actually broader than bspm as you and I discussed. It can arise whenever length(.libPaths()) > 1.