`package-installed-p` returns nil for installed (or built-in) packages.

In GNU Emacs 25.0.50.1 (x86_64-unknown-linux-gnu, GTK+ Version 2.24.23)
 of 2015-02-13 on scholc3-linux



Repository revision: a03ab7eaf532075d2948ece70b8f3c97cd26b577