Undo breaking changes in SwiftProtobufPluginLibrary #1433

Job	Run time
Api Breakage Compared to main Branch	1m 35s
Api Breakage Compared to Last Release	1m 22s
core (5.10.1-jammy, ref/heads/main)	13m 32s
fuzzing_regressions (debug)	3m 21s
core (5.9.2-jammy, SWIFT_BUILD_TEST_HOOK="-Xswiftc -warnings-as-errors", ref/heads/main)	1m 5s
fuzzing_regressions (release)	2m 50s
sanitizer_testing (address, debug)	2m 25s
core (5.8.1-jammy, SWIFT_BUILD_TEST_HOOK="-Xswiftc -warnings-as-errors", ref/heads/main)	1m 3s
sanitizer_testing (address, release)	5m 5s
sanitizer_testing (thread, debug)	2m 30s
sanitizer_testing (thread, release)	4m 23s
	39m 11s

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