Find a module’s source location
find_in_path(spec, base_paths)find_in_path returns a mod_info that specifies the
module source location.
a mod_spec.
a character vector of paths to search the module in, in order of preference.
A module is physically represented in the file system either by
‹spec_name(spec)›.r or by ‹spec_name(spec)›/__init__.r, in that
order of preference in case both exist. File extensions are case insensitive
to allow for R’s obsession with capital-R extensions (but lower-case are
given preference, and upper-case file extensions are discouraged).