Bug 18703: Add message to library() in library(include.only=) calls where that directive is redundant

[hturner]

As described in Bug 18703 this follows up on an R-devel thread discussing the behaviour of library() with the include.only argument. Three bugs (18703, 18704 and 18704) were created to implement follow-up actions. The task here is to add a message when a directive such as

library(Matrix, include.only="isDiagonal")

is redundant, because the Matrix package (or parts of it) are already loaded.

[hturner]

@mmaechler suggests on Bugzilla to use a classed message. An example of a classed message is given in the base::.packageStartupMessage() function:

    structure(list(message = message, call = call),
              class = c("packageStartupMessage",
                        "simpleMessage", "message", "condition"))

For an example of a classed warning, see grDevices:::warnLogCoords().

The actual message should use gettext() or gettextf() so the message is translated as required - see existing examples in the source code for library().