Rename package (and repo?) to flysystem-nextcloud-internal

[Potherca]

Currently, the name of this repo (and package) could lead to confusion, as this adapter is not meant to talk to Nextcloud externally (the WebDav adapter is better suited for that) but to use the FlySystem API within Nextcloud.

To remedy this confusion, the package should be renamed to flysystem-nextcloud-internal or something similar.

We could also rename this repo but I am not (yet) convinced that is really needed...