dir(), but without dot-prefix
dir(), you can set
full.names = FALSE, but then you can only
pass a character vector of length one as
path to not loose the information
about where the files are. This function solves that case. It's needed when
one wants to standardize paths to use set operations on them, i.e. when the
user supplied input does not have a dot prefix. See 'Examples'.
setdiff("./file.R", "file.R") # you want to standardize first.#>  "./file.R"