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.
By sanitizing the home file name in the derivation name, the home file
name is no longer exposed to the naming restrictions for nix store
paths.
For example, it is now possible to define home files with spaces in
their names without providing a target or source attribute.
