Returns the relative, descendent directory path between this module and other.
Throws if no such path exists.
For example, if module mod1 has path /dir1/mod1.pkl, and module mod2 has path /dir1/dir2/dir3/mod2.pkl,
then mod1.relativePathTo(mod2) will return List("dir2", "dir3").
A common use case is to compute the directory path between a template located at the root of a hierarchy
(say rootModule.pkl) and the currently evaluated module (accessible via the module keyword):
Generates Swift sources from Pkl