Unfortunately systemd derives nonsensical unit names when the unit
file is a link to a link to a file. This commit ensures that any file
whose target path matches the pattern `*/systemd/user/*` will be
reachable with only one link hop.
This also reverts f52ec0df7c, which
contained a temporary fix. This commit is an improvements in that it
is more explicit and also handles unit files given directly as a home
file source.
7a9c873093