[R] How to source files from a search path?

[blue sky]

Suppose some environment variable (say MY_R_INC) has a number of
paths. I want to source some file relative to some path in $MY_R_INC
(just as #include in C++ does, which looks for header file in a number
of directories). I RSiteSearch'ed, but I don't find any function that
satisfies my need. Could somebody let me know if I overlooked
something?