utils: prefer parent uri when matching uri

[mnauw]

At present, match_uri_to_workspace can return a workspace that is not a parent of the uri. In particular, if /a/b/c is a workspace, and also /a/b, then searching for a workspace for /a/b/x/y can return either one of the previous. In this case, however, /a/b is preferred as it is actually a parent (whereas /a/b/c is not). While this may be an "odd" workspace setup, it can occur when dealing with git submodules or some other project setup (also depending on the LSP client). But presumably it is preferable to be robust at the LSP server end than to declare such invalid.