cp: move copy_directory() to its own module

[jfinkels]

This pull request refactors copy_directory() and related helper functions into its own new module copydir.rs. This commit also adds some additional structs and helper functions to make the code easier to read and maintain. This commit does not change the behavior of the copy_directory() function, only the organization of the code.

I am proposing this change because I felt while working on pull request #3894 that it was harder than necessary for me to read and understand the code.