bazel: Add helper for maven_install's override_targets

This can be used to prevent duplicate classes in the classpath, one via
Maven and one via Bazel-native.

See census-instrumentation/opencensus-java#1963 and #5359

core/BUILD.bazel

@@ -63,3 +63,17 @@ java_library(
     ],
 )
 
+# Mirrors the dependencies included in the artifact on Maven Central for usage
+# with maven_install's override_targets. Purposefully does not export any
+# symbols, as it should only be used as a dep for pre-compiled binaries on
+# Maven Central.
+java_library(
+    name = "core_maven",
+    visibility = ["//visibility:public"],
+    runtime_deps = [
+        ":inprocess",
+        ":internal",
+        ":util",
+        "//api",
+    ],
+)

netty/BUILD.bazel

@@ -28,10 +28,13 @@ java_library(
     ],
 )
 
-# Not actually shaded, but mirrors the shaded dependencies included in the
+# Mirrors the dependencies included in the artifact on Maven Central for usage
-# artifact on Maven Central for usage with maven_install's override_targets.
+# with maven_install's override_targets. Purposefully does not export any
+# symbols, as it should only be used as a dep for pre-compiled binaries on
+# Maven Central. Not actually shaded; libraries should not be referencing
+# unstable APIs so there should not be any references to the shaded package.
 java_library(
-    name = "shaded",
+    name = "shaded_maven",
     visibility = ["//visibility:public"],
-    exports = ["//netty/shaded"],
+    runtime_deps = ["//netty/shaded"],
 )

repositories.bzl

@@ -3,6 +3,40 @@
 load("@bazel_tools//tools/build_defs/repo:http.bzl", "http_archive")
 load("@bazel_tools//tools/build_defs/repo:jvm.bzl", "jvm_maven_import_external")
 
+# For use with maven_install's override_targets.
+# maven_install(
+#     ...
+#     override_targets = IO_GRPC_GRPC_JAVA_OVERRIDE_TARGETS,
+# )
+#
+# If you have your own overrides as well, you can use:
+#     override_targets = dict(
+#         IO_GRPC_GRPC_JAVA_OVERRIDE_TARGETS,
+#         "your.target:artifact": "@//third_party/artifact",
+#     )
+#
+# To combine OVERRIDE_TARGETS from multiple libraries:
+#     override_targets = dict(
+#         IO_GRPC_GRPC_JAVA_OVERRIDE_TARGETS.items() +
+#         OTHER_OVERRIDE_TARGETS.items(),
+#         "your.target:artifact": "@//third_party/artifact",
+#     )
+IO_GRPC_GRPC_JAVA_OVERRIDE_TARGETS = {
+    "io.grpc:grpc-alts": "@io_grpc_grpc_java//alts",
+    "io.grpc:grpc-api": "@io_grpc_grpc_java//api",
+    "io.grpc:grpc-auth": "@io_grpc_grpc_java//auth",
+    "io.grpc:grpc-context": "@io_grpc_grpc_java//context",
+    "io.grpc:grpc-core": "@io_grpc_grpc_java//core_maven",
+    "io.grpc:grpc-grpclb": "@io_grpc_grpc_java//grpclb",
+    "io.grpc:grpc-netty": "@io_grpc_grpc_java//netty",
+    "io.grpc:grpc-netty-shaded": "@io_grpc_grpc_java//netty:shaded_maven",
+    "io.grpc:grpc-okhttp": "@io_grpc_grpc_java//okhttp",
+    "io.grpc:grpc-protobuf": "@io_grpc_grpc_java//protobuf",
+    "io.grpc:grpc-protobuf-lite": "@io_grpc_grpc_java//protobuf-lite",
+    "io.grpc:grpc-stub": "@io_grpc_grpc_java//stub",
+    "io.grpc:grpc-testing": "@io_grpc_grpc_java//testing",
+}
+
 def grpc_java_repositories(
         omit_bazel_skylib = False,
         omit_com_google_android_annotations = False,