the general rule is, that the module name you have to use in your Makefile is the same than the folder name of the implementation, e.g. USEMODULE += saul_reg means to include RIOT/sys/saul_reg/* to your project, or USEMODULE += dht includes the drivers/dht/* temp sensor driver. There are however some exceptions: networking modules you find under RIOT/sys/net for example. Further we have a number of pseudo-modules, being basically empty modules that have no code connected to them directly, but pull in dependencies. Have a look at RIOT/Makefile.pseudomoules for a list of those.