function posix(path) in designspaceLib does not work with Windows UNC path

On Windows UNC (Uniform Naming Convention) paths have the following form:
\\SERVER\SHARE\FOLDER-NAME\FILE-NAME or
\\IP-ADDRESS\SHARE\FOLDER-NAME\FILE-NAME
the possix function translates this to SERVER/SHARE/FOLDER-NAME/FILE-NAME. Therefore all path attributes for sources and instances are invalid if a DesignSpaceDocument is loaded via an UNC path.

[anthrotype]

The designspace spec says only relative paths are allowed, so that would basically exclude UNC paths

filename: required, string. A path to the source file, relative to the root path of this document. The path can be at the same level as the document or lower.

[anthrotype]

I meged #2306, thank you