"pkg prefix" is noisy

[Thomas Weber]

Hi, 

with current tip (3c16e2414564):

===============================
octave:1> pkg prefix /tmp
ans = /tmp
===============================

This is due to line 318, 

        local_packages = prefix;

which is later returned. I spent some time looking through my code for a
forgotten semicolon, until I understood that this comes from pkg.m
itself. 

So, is the above assignement necessary for something? If not, I'll
sumbmit a patch to remove it, its information value is zero.

        Thomas