drop-in compatibility adjustments

bump copied generated code to 0.12.1, the last version that had the old package names (includes KtGeneratedFileDescriptor annotation)
remove old protokt.proto file so that buf is happy that only one file has the protokt extension number
consolidate jvm extensions modules into multiplatform modules (puts files back in those JARs that were there before the 1.0 reorganization)
rename protokt outer class to be consistent with protobuf-java
remove kotlin-reflect from dependencies of protokt-core
remove interop between old and new generated code, as it is not needed for old generated code to function with the new runtime
undo manual adjustments to copied generated code as part of the above
kapt in multiplatform modules fails with: /Users/.../protokt/extensions/protokt-extensions-simple/build/tmp/kapt3/stubs/main/com/toasttab/protokt/ext/LocalDateStringConverter.java:3: error: incompatible types: NonExistentClass cannot be converted to Annotation @error.NonExistentClass(). Not sure what's up.

open-toast / protokt