Revert accidental change to submodule intent in "Add Mutable classes and ReadOnly capabilities

This reverts commit 02f1bdb.

# Conflicts:
#	compiler/src/dotty/tools/dotc/cc/CaptureOps.scala
#	compiler/src/dotty/tools/dotc/cc/CaptureRef.scala
#	compiler/src/dotty/tools/dotc/cc/CaptureSet.scala
#	compiler/src/dotty/tools/dotc/cc/CheckCaptures.scala
#	compiler/src/dotty/tools/dotc/cc/Existential.scala
#	compiler/src/dotty/tools/dotc/cc/Setup.scala
#	compiler/src/dotty/tools/dotc/core/Definitions.scala
#	compiler/src/dotty/tools/dotc/printing/PlainPrinter.scala
#	compiler/src/dotty/tools/dotc/printing/RefinedPrinter.scala
#	compiler/src/dotty/tools/dotc/typer/Checking.scala
#	library/src/scala/caps.scala
#	project/MiMaFilters.scala
#	tests/neg-custom-args/captures/i21614.check
#	tests/neg-custom-args/captures/mut-outside-mutable.check
#	tests/neg-custom-args/captures/readOnly.check
#	tests/neg-custom-args/captures/sepchecks.scala

Author: odersky