ClassFileInstaller can't be used by classes outside of default package

ClassFileInstaller is in the default package hence it's impossible to use it directly by classes in any other package. although ClassFileInstaller is mainly used directly from jtreg test description, there are cases when one needs to use it in another "driver" class (e.g. to reduce boilerplate), yet to do. that these driver classes have to either be in a default package or use reflection, both approaches have drawbacks.

to solve that, we can move ClassFileInstaller implementation to a package, and to avoid unneeded churn, keep package-less ClassFileInstaller class which will call package-full impl.