Use `--prefix ""` - no prefix in modules

[tlaurion]

          Use `--prefix ""` - no prefix.  `--prefix "/"` causes funny-looking `//lib`, `//bin`, etc. to appear in various places, which don't harm anything but do look odd, and we shouldn't continue making the same mistake IMO.

Originally posted by @JonathonHall-Purism in https://github.com/osresearch/heads/pull/1446#discussion_r1290239297