Node Wrangler: Fix #104555, multiple operators using virtual sockets
The bpy_extras.node_utils.connect_sockets() API allows creating node
links to or from virtual sockets. It was used in
0bfeb38c41 to fix one
mode of the lazy connect operator, but the mode with menu was still
affected.
Other operators were affected too, such as Add Texture Setup, Merge
Nodes, or Swap Links when one of the input nodes was a Group Input
with no socket.
This commit replaces all occurrences of `links.new()` by
`bpy_extras.node_utils.connect_sockets()` in Node Wrangler. It should
have no drawback since the new API is compatible with the ordinary
one.