Keep repository paths system independent (or POSIX compatible) in create-sibling-ria and ora-remote code

[christian-monch]

The current implementation of create-sibling-ria and ora-remote assumes at different points that pathlib.Path on then client-side creates paths that can be used to identify resources in the RIA-store and can therefore be used to operate on RIA-store paths, e.g. assemble the path to ria-layout-version. This is not true in general.

It is only true if the RIA-store is located on the local file system. It is not true if the RIA-store is remote and, for example, accessed via a ssh-URL. In the case of a remote RIA-store the path-type can be different from the client machine

Because the current remote implementations are accessed via URLs and the URL path component is in POSIX style and because we currently only support remote RIA-stores on POSIX systems, the implementation will fail when accessing a remote RIA-store from a Windows client.

Until this issue is resolved, no Windows client will work with remote RIA-stores.

[christian-monch]

Fixed by PR #669