The modules that I've downloaded and looked at do not include "pike/" in their #includes. Is there an incompatibility between 7.6 and 7.8, or are there different rules for modules than for other code?
7.6 does not provide the "simplified" module build mechanism, so there are no 7.6 modules which use this. Frankly, I don't know of any 7.8 modules that do either, which ones did you find?