This was almost certainly a deliberate and paid-for 'product placement' by the filmmakers. Along with the Nestlé bar, there were also prominent and unsubtle placements of Coca Cola, Perrier, Cheez-its, Budweiser beer, real-world magazines, newspapers and pharmaceuticals ("Do you have any Excedrin or extra strength Tylenol?", "That's a very big twinkie", etc).
Interestingly, in the two official novelisations the bar was changed to a Baby Ruth in one, and unnamed in the other.
Venkman reached into his pocket. “Egon, I take back every bad thing I ever said about you. Here.” He held up a candy bar. Egon smiled
delightedly and reached for it, but Venkman pulled it back. They
looked at each other for a moment, then Venkman pressed it into his
hands. “You earned it . . .”
“Baby Ruth,” Spengler said reverently, ripping off the paper and
cramming it into his face. “Gooomph!”
In the comic series, Egon's favourite bar is replaced with a "chomp" bar, identical in size, shape, colouration and font to the Nestlé product but with a slightly different (and generic) name.