Make ObjectAdapter package-protected

Make the ObjectAdapter class package-protected by removing the
dependency on it from some test classes. Since the ObjectAdapter is the
only complex class in the default storage package, the package can be
made public and accessible without a factory in a future commit.
Move default storage implementations out of .def

Move the default storage implementations (StandardStorage and
ReferenceStorage) out of the .def package. That means that no factory is
needed for creating instances of the storage classes.
Rename AbstractFunction to DefaultFunction

Rename the AbstractFunction class to DefaultFunction since the class is
not abstract.
Cut Abstract(Collection)Prop.Def. -> FunctionDef.Factory dependency

Cut the dependency from the Abstract(Collection)PropertyDefinition
classes to the FunctionDefinitionFactory class by introducing the
InternalDefaultFunctionDefinition class.
Replace FunctionDefinitionFactory utility with ClassmodFactory entry

Remove the old FunctionDefinitionFactory utility class and replace it
with a mapping for ClassmodFactory. Note that the new method doesn't
support the setting of a default function executor.
Fix wrong naming issue in DefaultCollectionPropertyDefinitionFactory

DefaultCollectionPropertyDefinitionFactory no longer mistakes "property"
for "collection property".
Make factory implementations package-protected

Make the factory implementations in the root package package-protected
in order to prevent access from outside.
Move Classmod class to root package

Move the Classmod class to the root package. The class currently only
contains the CONTEXT_PATH constant.