loxi_frontend: move classes only used by c to c_gen

 - flags
 - identifiers
 - match
 - translation
 - type_maps

were only used by the c backend. I'm moving them to c_gen to
unclutter the loxi_front_end space (and updating references
in the c backend)
diff --git a/c_gen/templates/locitest/test_validator.c b/c_gen/templates/locitest/test_validator.c
index 6b9dd87..eb3cbb6 100644
--- a/c_gen/templates/locitest/test_validator.c
+++ b/c_gen/templates/locitest/test_validator.c
@@ -27,8 +27,8 @@
 ::
 :: include('_copyright.c')
 :: import of_g
-:: from loxi_utils import loxi_utils
-:: from loxi_front_end import type_maps
+:: import c_gen.loxi_utils_legacy as loxi_utils
+:: from c_gen import type_maps
 
 /**
  * Test message validator