gnulib-tool: Alternative structure of testdirs, similar to --import.

gnulib-tool: Alternative structure of testdirs, similar to --import.

* gnulib-tool: New option --single-configure.
(func_usage): Document it.
(single_configure): New variable.
(func_modules_transitive_closure_separately,
func_modules_transitive_closure_separately,
func_determine_use_libtests, func_modules_add_dummy_separately,
func_modules_to_filelist_separately): New functions, extracted from
func_import.
(func_emit_tests_Makefile_am): Handle $single_configure = true case.
(func_import): Use the new functions.
(func_create_testdir): Set final_modules. Handle $single_configure =
true case.