modules: store module name in the module struct.

* m4/m4private.h (m4_module): Add name field.
* m4/module.c (m4__module_open): Save the name field.
* m4/path.c (m4_load_filename): Pass the raw filename to
m4_module_load.
* m4/module.c (m4_get_module_name): Replace all the ltdl
twiddling with returning the saved name field.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>