Move `compiler/plugin.pb.cc` to libprotobuf with the other WKT sources.

This lets all values of `WELL_KNOWN_PROTOS` be treated the same with
regard to Bazel's protobuf blacklisting.

BUILD

@@ -121,6 +121,7 @@ cc_library(
         "src/google/protobuf/api.pb.cc",
         "src/google/protobuf/compiler/importer.cc",
         "src/google/protobuf/compiler/parser.cc",
+        "src/google/protobuf/compiler/plugin.pb.cc",
         "src/google/protobuf/descriptor.cc",
         "src/google/protobuf/descriptor.pb.cc",
         "src/google/protobuf/descriptor_database.cc",
@@ -378,7 +379,6 @@ cc_library(
         "src/google/protobuf/compiler/objectivec/objectivec_primitive_field.cc",
         "src/google/protobuf/compiler/php/php_generator.cc",
         "src/google/protobuf/compiler/plugin.cc",
-        "src/google/protobuf/compiler/plugin.pb.cc",
         "src/google/protobuf/compiler/python/python_generator.cc",
         "src/google/protobuf/compiler/ruby/ruby_generator.cc",
         "src/google/protobuf/compiler/subprocess.cc",

cmake/libprotobuf.cmake

@@ -4,6 +4,7 @@ set(libprotobuf_files
   ${protobuf_source_dir}/src/google/protobuf/api.pb.cc
   ${protobuf_source_dir}/src/google/protobuf/compiler/importer.cc
   ${protobuf_source_dir}/src/google/protobuf/compiler/parser.cc
+  ${protobuf_source_dir}/src/google/protobuf/compiler/plugin.pb.cc
   ${protobuf_source_dir}/src/google/protobuf/descriptor.cc
   ${protobuf_source_dir}/src/google/protobuf/descriptor.pb.cc
   ${protobuf_source_dir}/src/google/protobuf/descriptor_database.cc

cmake/libprotoc.cmake

@@ -88,7 +88,6 @@ set(libprotoc_files
   ${protobuf_source_dir}/src/google/protobuf/compiler/objectivec/objectivec_primitive_field.cc
   ${protobuf_source_dir}/src/google/protobuf/compiler/php/php_generator.cc
   ${protobuf_source_dir}/src/google/protobuf/compiler/plugin.cc
-  ${protobuf_source_dir}/src/google/protobuf/compiler/plugin.pb.cc
   ${protobuf_source_dir}/src/google/protobuf/compiler/python/python_generator.cc
   ${protobuf_source_dir}/src/google/protobuf/compiler/ruby/ruby_generator.cc
   ${protobuf_source_dir}/src/google/protobuf/compiler/subprocess.cc

src/Makefile.am

@@ -278,6 +278,7 @@ libprotobuf_la_SOURCES =                                       \
   google/protobuf/io/zero_copy_stream_impl.cc                  \
   google/protobuf/compiler/importer.cc                         \
   google/protobuf/compiler/parser.cc                           \
+  google/protobuf/compiler/plugin.pb.cc                        \
   google/protobuf/util/delimited_message_util.cc               \
   google/protobuf/util/field_comparator.cc                     \
   google/protobuf/util/field_mask_util.cc                      \
@@ -333,7 +334,6 @@ libprotoc_la_SOURCES =                                         \
   google/protobuf/compiler/code_generator.cc                   \
   google/protobuf/compiler/command_line_interface.cc           \
   google/protobuf/compiler/plugin.cc                           \
-  google/protobuf/compiler/plugin.pb.cc                        \
   google/protobuf/compiler/subprocess.cc                       \
   google/protobuf/compiler/subprocess.h                        \
   google/protobuf/compiler/zip_writer.cc                       \