The Dune prequels say that people had used computers to navigate before the Butlerian Jihad. They only became dependent on the spice when they outlawed computers.
But what about the original series? On Ix, computer navigators are invented as if they were novel. So how did people ever get to Arrakis to get the spice in the first place?
After the Butlerian Jihad and the outlawing of computering devices, Spice from Arrakis became the key element for activating the psychic abilities to successfully navigate using the problematic Holtzman Effect drive.
By the time Ix was investigating them, navigational systems WERE novel to Ix, having had centuries of no computerized technology to assist navigation, it would have been as if they had just been discovered.
Space travel came in two major varieties in the Dune Universe: Conventional FTL and Holtzman Effect Space Folding:
Conventional FTL: Space travel was used mainly for travel within the confines of a star system (not for interstellar travel). However, before the discovery of the new faster-than-light travel method, it was also used for long-distance space travel. The old method was described as "outraceing photons". Even after space-folding became the primary means of interstellar travel, many Imperial warships still kept their old FTL drives as an alternative to the much faster but less reliable Holtzmann engines.
Holtzman Effect Space Folding: This form of travel, while extremely expensive, was also not safe as one in ten ships that used space folding engine disappeared, at least during the early years of the technology's use before the advent of Navigators. It was utilized for both commercial and military purposes. Space-folding made use of two key factors:
The folding of space-time, which was accomplished by the practical application of the Holtzman Effect.
The plotting of a safe course through folded space by a Guild Navigator, who used prescient planning induced by vast amounts of the Spice Melange.
REF: Dune Wikia; The Scattering, Arrakis, Space Travel, The Spacing Guild
For Ix, interstellar navigation was being rediscovered. Remember, after the Butlerian Jihad, anything that used any serious computer technology, particularly artificial intelligence was destroyed and information suppressed or erased.
It was after the fall of computerization that interstellar travel became the sole province of the Spacer's Guild. Before that, Mankind was able to spread through the galaxy through the use of sophisticated computer technology.
The only clue I know of in the original Frank Herbert-written books is in Appendix II, the religion appendix in Dune:
Mankind's movement through deep space placed a unique stamp on religion during the one hundred and ten centuries that preceded the Butlerian Jihad. To begin with, early space travel, although widespread, was largely unregulated, slow, and uncertain, and, before the Guild monopoly, was accomplished by a hodgepodge of methods. The first space experiences, poorly communicated and subject to extreme distortion, were a wild inducement to mystical speculation.
So there doesn't appear to be any exact canon answer. But the idea appears to be that early space travellers experimented with FTL drives, wormholes, cryogenic colonization ships, and whatever else they could manage before the invention of the Foldspace drive.
There were certainly other drugs used by Navigators before the Spice.
In fact, it is heavily implied in Dune that the use of Spice by both the Guild and the Bene Gesserit is a very recent thing: perhaps no more than a hundred years, but in any case definitely much more recent than the Butlerian Jihad. Three pieces of evidence:
The Dune Encyclopedia (effectively published fanfiction, apocryphal but agreeing with Frank's original books) notes the finding of Arrakis as one of life's great mysteries: "without Spice, no Space; without Space, no Spice". It is highly unlikely that, after the Butlerians shut down FTL navigation, someone funded enough expeditions to find Arrakis by chance.
My own pet theory is that Arrakis had been found before, and the discoverers somehow found the small-scale prescient effects of consuming spice. Rather than reveal the planet and get a fat finder's bonus, they made more money with their predictive skills, either in casinos or day-trading or the like. The fall of computerized navigation cut off whoever from that group was actually on Arrakis, and in desperation they did some massive spice dosing to predict their way out of their situation.
