simplify WP directory renaming in output; force FQURLs in exported site to aid custom rewriting

Current *solution* is overly complicated and error prone.

Plugin knows the locations of these key WP directories, so the user should just be able to input against each one, ie:

```
wp-content -> new-wp-content
themes -> new-themes
parent-theme -> new-parent-theme
child-theme -> new-child-theme
plugins -> new-plugins
inc -> new inc
[list of plugin names] do similar
```