[Bazel] Add back a filegroup for :well_known_protos. (#10061)

This was removed in #9915, since it is misleadingly named (the set of sources is "well known," but not the same as the Well-Known Types).

BUILD.bazel

@@ -128,6 +128,23 @@ cc_library(
     visibility = ["//visibility:public"],
 )
 
+# Source protos that are typically part of the protobuf runtime.
+#
+# DEPRECATED: Prefer :well_known_type_protos for the Well-Known Types
+# (https://developers.google.com/protocol-buffers/docs/reference/google.protobuf)
+# or :descriptor_proto(_srcs) for descriptor.proto (source), or
+# :compiler_plugin_proto for compiler/plugin.proto.
+filegroup(
+    name = "well_known_protos",
+    srcs = [
+        ":descriptor_proto_srcs",
+        ":well_known_type_protos",
+        "//src/google/protobuf/compiler:plugin.proto",
+    ],
+    deprecation = "Prefer :well_known_type_protos instead.",
+    visibility = ["//visibility:public"],
+)
+
 ################################################################################
 # Protocol Buffers Compiler
 ################################################################################

src/google/protobuf/compiler/BUILD.bazel

@@ -150,6 +150,11 @@ filegroup(
     ],
 )
 
+exports_files(
+    srcs = ["plugin.proto"],
+    visibility = ["//:__pkg__"],
+)
+
 cc_library(
     name = "mock_code_generator",
     testonly = 1,