Move FileTransfer to the transfers package
Refactor from SymlinkFT AbstractExecFT
Introduce afterSuccess in the FileTransfer interface

A simple `mv` command is not possible since 1) the file
must be present for subsequent readds and 2) it's not
safe to remove the file until the import has completed
successfully.

Now, users of the `FileTransfer` API have the responsibility
of calling `afterSuccess` once the `ImportLibrary` has done
its work. An exception may be thrown if cleanup is not
possible.
Add simple package-info.java