bazel: Add proto_library rules for well known types. Fixes #2763

Adds a proto_library rule for each well known type proto:

$ bazel query "filter(\".*_proto$\", \"...\")"
//:wrappers_proto
//:timestamp_proto
//:struct_proto
//:field_mask_proto
//:empty_proto
//:duration_proto
//:compiler_plugin_proto
//:descriptor_proto
//:api_proto
//:type_proto
//:source_context_proto
//:any_proto

Bazel users can reference these proto_library rules for their own
language specific rules i.e.

java_proto_library(
  name = "any_java_proto",
  deps = ["@com_google_protobuf//:any_proto"],
)

Also set the workspace name to "com_google_protobuf", as proto_library
rules reference protobuf that way.

BUILD

@@ -224,21 +224,24 @@ objc_library(
     visibility = ["//visibility:public"],
 )
 
-RELATIVE_WELL_KNOWN_PROTOS = [
-    # AUTOGEN(well_known_protos)
-    "google/protobuf/any.proto",
-    "google/protobuf/api.proto",
-    "google/protobuf/compiler/plugin.proto",
-    "google/protobuf/descriptor.proto",
-    "google/protobuf/duration.proto",
-    "google/protobuf/empty.proto",
-    "google/protobuf/field_mask.proto",
-    "google/protobuf/source_context.proto",
-    "google/protobuf/struct.proto",
-    "google/protobuf/timestamp.proto",
-    "google/protobuf/type.proto",
-    "google/protobuf/wrappers.proto",
-]
+# Map of all well known protos.
+# name => (include path, imports)
+WELL_KNOWN_PROTO_MAP = {
+    "any" : ("google/protobuf/any.proto", []),
+    "api" : ("google/protobuf/api.proto", ["source_context", "type"]),
+    "compiler_plugin" : ("google/protobuf/compiler/plugin.proto", ["descriptor"]),
+    "descriptor" : ("google/protobuf/descriptor.proto", []),
+    "duration" : ("google/protobuf/duration.proto", []),
+    "empty" : ("google/protobuf/empty.proto", []),
+    "field_mask" : ("google/protobuf/field_mask.proto", []),
+    "source_context" : ("google/protobuf/source_context.proto", []),
+    "struct" : ("google/protobuf/struct.proto", []),
+    "timestamp" : ("google/protobuf/timestamp.proto", []),
+    "type" : ("google/protobuf/type.proto", ["any", "source_context"]),
+    "wrappers" : ("google/protobuf/wrappers.proto", []),
+}
+
+RELATIVE_WELL_KNOWN_PROTOS = [proto[1][0] for proto in WELL_KNOWN_PROTO_MAP.items()]
 
 WELL_KNOWN_PROTOS = ["src/" + s for s in RELATIVE_WELL_KNOWN_PROTOS]
 
@@ -258,6 +261,33 @@ cc_proto_library(
     visibility = ["//visibility:public"],
 )
 
+################################################################################
+# Well Known Types Proto Library Rules
+#
+# These proto_library rules can be used with one of the language specific proto
+# library rules i.e. java_proto_library:
+#
+# java_proto_library(
+#   name = "any_java_proto",
+#   deps = ["@com_google_protobuf//:any_proto],
+# )
+################################################################################
+
+internal_copied_filegroup(
+    name = "_internal_wkt_protos",
+    srcs = WELL_KNOWN_PROTOS,
+    dest = "",
+    strip_prefix = "src",
+    visibility = ["//visibility:hidden"],
+)
+
+[proto_library(
+    name = proto[0] + "_proto",
+    srcs = [proto[1][0]],
+    deps = [dep + "_proto" for dep in proto[1][1]],
+    visibility = ["//visibility:public"],
+    ) for proto in WELL_KNOWN_PROTO_MAP.items()]
+
 ################################################################################
 # Protocol Buffers Compiler
 ################################################################################

WORKSPACE

@@ -1,3 +1,5 @@
+workspace(name = "com_google_protobuf")
+
 new_git_repository(
     name = "googletest",
     build_file = "gmock.BUILD",