Create toolchain types for proto_library and lang_proto_library

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Create toolchain types for proto_library and lang_proto_library

This handles only lang_proto_libraries defined in Bazel: Java, Java lite, Python, C++.

Design doc: https://docs.google.com/document/d/1CE6wJHNfKbUPBr7-mmk_0Yo3a4TaqcTPE0OWNuQkhPs/edit#heading=h.5mcn15i0e1ch
Issue: https://github.com/bazelbuild/rules_proto/issues/179
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PiperOrigin-RevId: 566380737
Change-Id: I71dd30ed45c0f1d727600989ed1e50e9e5d70411

java/proto/BUILD

diff --git a/java/proto/BUILD b/java/proto/BUILD
new file mode 100644
index 0000000..c6b4b50
--- /dev/null
+++ b/java/proto/BUILD
@@ -0,0 +1,7 @@
+package(default_visibility = ["//visibility:public"])
+
+# Toolchain type provided by proto_lang_toolchain rule and used by java_proto_library
+toolchain_type(name = "toolchain_type")
+
+# Toolchain type provided by proto_lang_toolchain rule and used by java_lite_proto_library
+toolchain_type(name = "lite_toolchain_type")