I just downloaded moodle-weekly-19.tgz to make sure there isn't some anomaly in it as you seem to be describing. I found it extracts to a folder (directory) called moodle-weekly-19.tgz, which contains only the folder called moodle, which is the main Moodle folder. It does indeed contain a folder called lib, as Moodle always has (at least since I've been involved with it, late 2006), not libdir.
Then I opened install.php to see if it was looking for a folder called libdir. I did a thorough search of the file in my text editor for the string "libdir" and found the only occurrences were in the object property $CFG->libdir, which is set equal to the value of $CFG->dirroot
, the pathname of the main Moodle folder, followed by "/lib", not "/libdir", just as Moodle normally does.
So if what I have said is not true of the download
file you got, it has been fixed. Try the download again.