[ObjC] Small cleanups to package level headers

- Reduce what headers the runtime import needs.
- Remove stale function prototype.
- Move one function to be full scoped to a single file.

PiperOrigin-RevId: 512073294

objectivec/GPBMessage.m

@@ -47,6 +47,12 @@
 #import "GPBUnknownFieldSet_PackagePrivate.h"
 #import "GPBUtilities_PackagePrivate.h"
 
+// Returns a new instance that was automatically created by |autocreator| for
+// its field |field|.
+static GPBMessage *GPBCreateMessageWithAutocreator(Class msgClass, GPBMessage *autocreator,
+                                                   GPBFieldDescriptor *field)
+    __attribute__((ns_returns_retained));
+
 // Direct access is use for speed, to avoid even internally declaring things
 // read/write, etc. The warning is enabled in the project to ensure code calling
 // protos can turn on -Wdirect-ivar-access without issues.
@@ -827,8 +833,8 @@ static void ExtensionMergeFromInputStream(GPBExtensionDescriptor *extension, BOO
   }
 }
 
-GPBMessage *GPBCreateMessageWithAutocreator(Class msgClass, GPBMessage *autocreator,
-                                            GPBFieldDescriptor *field) {
+static GPBMessage *GPBCreateMessageWithAutocreator(Class msgClass, GPBMessage *autocreator,
+                                                   GPBFieldDescriptor *field) {
   GPBMessage *message = [[msgClass alloc] init];
   message->autocreator_ = autocreator;
   message->autocreatorField_ = [field retain];

objectivec/GPBMessage_PackagePrivate.h

@@ -86,15 +86,6 @@ typedef struct GPBMessage_Storage *GPBMessage_StoragePtr;
 
 CF_EXTERN_C_BEGIN
 
-// Call this before using the readOnlySemaphore_. This ensures it is created only once.
-void GPBPrepareReadOnlySemaphore(GPBMessage *self);
-
-// Returns a new instance that was automatically created by |autocreator| for
-// its field |field|.
-GPBMessage *GPBCreateMessageWithAutocreator(Class msgClass, GPBMessage *autocreator,
-                                            GPBFieldDescriptor *field)
-    __attribute__((ns_returns_retained));
-
 // Returns whether |message| autocreated this message. This is NO if the message
 // was not autocreated by |message| or if it has been mutated since
 // autocreation.

objectivec/GPBProtocolBuffers_RuntimeSupport.h

@@ -36,7 +36,6 @@
 // clang-format on
 
 #import "GPBDescriptor_PackagePrivate.h"
-#import "GPBExtensionInternals.h"
-#import "GPBMessage_PackagePrivate.h"
+#import "GPBMessage.h"
 #import "GPBRootObject_PackagePrivate.h"
 #import "GPBUtilities_PackagePrivate.h"