Use repo-relative labels wherever possible (#9187)

* Use repo-relative labels wherever possible

The label `@com_google_protobuf//:foo` within the protobuf repo is often synonymous with just `//:foo`. We should prefer the latter as it allows us to use a shorter name for the module in the Bazel Central Registry (so just "protobuf" instead of "com_google_protobuf").

Note that the semantics can be subtle: in a macro, plain strings are anchored to the *calling* repo, so if we just use `//:foo` as the default value of a macro argument, it will be resolved to `@myrepo//:foo` if the macro is called from the repo `@myrepo`. In this case, it's necessary to directly call the `Label()` constructor to anchor the string label to the repo where the .bzl file lives.

See https://github.com/bazelbuild/bazel-central-registry/pull/28#issuecomment-954741081 for a bit more context.

* fix protobuf_deps.bzl

protobuf.bzl

@@ -259,9 +259,9 @@ def cc_proto_library(
         deps = [],
         cc_libs = [],
         include = None,
-        protoc = "@com_google_protobuf//:protoc",
+        protoc = Label("//:protoc"),
         use_grpc_plugin = False,
-        default_runtime = "@com_google_protobuf//:protobuf",
+        default_runtime = Label("//:protobuf"),
         **kargs):
     """Bazel rule to create a C++ protobuf library from proto source files
 
@@ -379,7 +379,7 @@ internal_gen_well_known_protos_java = rule(
         "_protoc": attr.label(
             executable = True,
             cfg = "exec",
-            default = "@com_google_protobuf//:protoc",
+            default = "//:protoc",
         ),
     },
 )
@@ -422,8 +422,8 @@ def py_proto_library(
         py_libs = [],
         py_extra_srcs = [],
         include = None,
-        default_runtime = "@com_google_protobuf//:protobuf_python",
-        protoc = "@com_google_protobuf//:protoc",
+        default_runtime = Label("//:protobuf_python"),
+        protoc = Label("//:protoc"),
         use_grpc_plugin = False,
         **kargs):
     """Bazel rule to create a Python protobuf library from proto source files

protobuf_deps.bzl

@@ -29,7 +29,7 @@ def protobuf_deps():
     if not native.existing_rule("zlib"):
         http_archive(
             name = "zlib",
-            build_file = "@com_google_protobuf//:third_party/zlib.BUILD",
+            build_file = Label("//:third_party/zlib.BUILD"),
             sha256 = "629380c90a77b964d896ed37163f5c3a34f6e6d897311f1df2a7016355c45eff",
             strip_prefix = "zlib-1.2.11",
             urls = ["https://github.com/madler/zlib/archive/v1.2.11.tar.gz"],