Collapse :protobuf_objc and :objectivec targets.

Not sure the history for why there were two objc_library targets, but
given the one has an issue about not working, merge them into a single
target and an alias.

- Use the alias for :objectivec since that naming doesn't seem to follow
  the other targets *_library targets here.
- Update :protobuf_objc to be use for the working target and just inline
  the sources/headers lists so there is less indirection in the file.

Fixes #5284

BUILD

@@ -196,14 +196,6 @@ cc_library(
     visibility = ["//visibility:public"],
 )
 
-objc_library(
-    name = "protobuf_objc",
-    hdrs = ["objectivec/GPBProtocolBuffers.h"],
-    includes = ["objectivec"],
-    non_arc_srcs = ["objectivec/GPBProtocolBuffers.m"],
-    visibility = ["//visibility:public"],
-)
-
 # Map of all well known protos.
 # name => (include path, imports)
 WELL_KNOWN_PROTO_MAP = {
@@ -861,7 +853,15 @@ proto_lang_toolchain(
     visibility = ["//visibility:public"],
 )
 
-OBJC_HDRS = [
+alias(
+    name = "objectivec",
+    actual = ":protobuf_objc",
+    visibility = ["//visibility:public"],
+)
+
+objc_library(
+    name = "protobuf_objc",
+    hdrs = [
         "objectivec/GPBArray.h",
         "objectivec/GPBBootstrap.h",
         "objectivec/GPBCodedInputStream.h",
@@ -890,9 +890,8 @@ OBJC_HDRS = [
         "objectivec/google/protobuf/Timestamp.pbobjc.h",
         "objectivec/google/protobuf/Type.pbobjc.h",
         "objectivec/google/protobuf/Wrappers.pbobjc.h",
-]
-
-OBJC_PRIVATE_HDRS = [
+        # Package private headers, but exposed because the generated sources
+        # need to use them.
         "objectivec/GPBArray_PackagePrivate.h",
         "objectivec/GPBCodedInputStream_PackagePrivate.h",
         "objectivec/GPBCodedOutputStream_PackagePrivate.h",
@@ -903,9 +902,14 @@ OBJC_PRIVATE_HDRS = [
         "objectivec/GPBUnknownFieldSet_PackagePrivate.h",
         "objectivec/GPBUnknownField_PackagePrivate.h",
         "objectivec/GPBUtilities_PackagePrivate.h",
-]
-
-OBJC_SRCS = [
+    ],
+    copts = [
+        "-Wno-vla",
+    ],
+    includes = [
+        "objectivec",
+    ],
+    non_arc_srcs = [
         "objectivec/GPBArray.m",
         "objectivec/GPBCodedInputStream.m",
         "objectivec/GPBCodedOutputStream.m",
@@ -930,18 +934,7 @@ OBJC_SRCS = [
         "objectivec/google/protobuf/Timestamp.pbobjc.m",
         "objectivec/google/protobuf/Type.pbobjc.m",
         "objectivec/google/protobuf/Wrappers.pbobjc.m",
-]
-
-objc_library(
-    name = "objectivec",
-    hdrs = OBJC_HDRS + OBJC_PRIVATE_HDRS,
-    copts = [
-        "-Wno-vla",
-    ],
-    includes = [
-        "objectivec",
     ],
-    non_arc_srcs = OBJC_SRCS,
     visibility = ["//visibility:public"],
 )