According to the GTA Wikia, It's possible to collect all Hidden Packages before the first mission by using various exploits or tricks to reach the unlocked islands. I've already managed to jump the hole left in the Callahan Bridge and made it to Staunton Island.
Well, one package in particular is on the Lift Bridge and can only be reached when the bridge is raised, and the bridge only raises when Shoreside Vale is unlocked. I'm assuming this was just an oversight and the editor simply forgot this hidden package cannot be exploited, or is there another way?
On another note, I've read previous tutorials for missions which mention collecting all hidden packages and using the rocket launcher.