pull the mini descriptor encoders into their proper .c files

Performance neutral but it simplifies the code, shrinks the public surface,
and makes logical sense

PiperOrigin-RevId: 478038589
diff --git a/BUILD b/BUILD
index fd16a0a..7504a61 100644
--- a/BUILD
+++ b/BUILD
@@ -399,6 +399,8 @@
         "upb/reflection/def_pool.h",
         "upb/reflection/def_type.c",
         "upb/reflection/def_type.h",
+        "upb/reflection/desc_state.c",
+        "upb/reflection/desc_state.h",
         "upb/reflection/enum_def.c",
         "upb/reflection/enum_def.h",
         "upb/reflection/enum_value_def.c",
@@ -414,7 +416,6 @@
         "upb/reflection/message_def.h",
         "upb/reflection/method_def.c",
         "upb/reflection/method_def.h",
-        "upb/reflection/mini_descriptor_encode.c",
         "upb/reflection/oneof_def.c",
         "upb/reflection/oneof_def.h",
         "upb/reflection/service_def.c",
@@ -429,7 +430,6 @@
         "upb/reflection/def.hpp",
         "upb/reflection/message.h",
         "upb/reflection/message.hpp",
-        "upb/reflection/mini_descriptor_encode.h",
     ],
     copts = UPB_DEFAULT_COPTS,
     visibility = ["//visibility:public"],