… from proto_info

NEW: don't try to compute plugin_output automatically

Additional information is needed whether protoc generates a single file or multiple files.

Add output_files to proto_lang_toolchain (enum "single","multiple") and propagate it through ProtoLangToolchainInfo.

Add experimental_output_files to proto_common.compile, that can override the value, for faster migration path.

When the value is set, automatically compute plugin_output.

When the (legacy) plugin_output is not set to a file, set it automatically to correct directory.

AI: Cherry-pick this change to Bazel minor release and follow up with updates to proto_lang_toolchain.

Fixes: bazelbuild#18263
Tracking issue: bazelbuild#18623
PiperOrigin-RevId: 541964181
Change-Id: Ie4b4792287723798ffdd4047562d62eb05d1b731