> I guess we could try and check with `locate-library'.

that sounds reasonable.

going through package--builtins (Emacs trunk) I also noticed that some
packages from Emacs are not in the list, while others are. for example
window.el is missing, but winner.el is included. uniquify.el is missing
too. I don't really understand this concept.

I think that the old behavior to have package-installed-p return t if the
min-version specified was (0) and the package didn't specify a version was
appropriate. is there any reason not to keep that behavior?

steckerhalter