Find a module<U+2019>s source location
find_in_path(spec, base_paths)a mod_spec.
a character vector of paths to search the module in, in order of preference.
find_in_path returns a mod_info that specifies the
module source location.
A module is physically represented in the file system either by
<U+2039>spec_name(spec)<U+203A>.r or by <U+2039>spec_name(spec)<U+203A>/__init__.r, in that
order of preference in case both exist. File extensions are case insensitive
to allow for R<U+2019>s obsession with capital-R extensions (but lower-case are
given preference, and upper-case file extensions are discouraged).