use `load-module-suffixes' as a source of module suffixes.

* New `load-module-suffixes' variable:

- `load-suffixes' corresponds to elisp suffixes only
- `load-modules-suffixes' corresponds to modules suffixes only
- `get-load-suffixes' use both lists to build every possible suffixes

* Slightly simplify `load' by introducing 2 predicates:

- string_prefix_p (string, suffix)
  Return truef if STRING ends with SUFFIX.

- string_suffixes_p (string, suffix_list)
  Return true if STRING ends with any element of SUFFIX_LIST.