systemd: do not pass .wants fragment path to manager_load_unit

systemd: do not pass .wants fragment path to manager_load_unit

When loading units, sometimes we'd first encounter a unit from .wants or
.requires directory. A typical case would be when multi-user.target.wants/
contains a symlink to some unit. We would prepare to load this unit using
/etc/systemd/system/multi-user.target.wants/foo.service as the fragment
path. This is always wrong. Instead, let's use NULL as the path and let
manager_load_unit() figure out the path on its own.

Fixes #9921.

    path=0x5625ed9b01a0 "/usr/lib/systemd/system/local-fs.target.wants/systemd-remount-fs.service", e=0x0,
    _ret=0x7ffe64645000) at ../src/core/manager.c:1887
    name=0x5625ed9b01ce "systemd-remount-fs.service",
    path=0x5625ed9b01a0 "/usr/lib/systemd/system/local-fs.target.wants/systemd-remount-fs.service", e=0x0,
    _ret=0x7ffe64645000) at ../src/core/manager.c:1961
    name=0x5625ed9b01ce "systemd-remount-fs.service",
    path=0x5625ed9b01a0 "/usr/lib/systemd/system/local-fs.target.wants/systemd-remount-fs.service",
    add_reference=true, mask=UNIT_DEPENDENCY_FILE) at ../src/core/unit.c:2946
    dir_suffix=0x5625ebb179ed ".wants") at ../src/core/load-dropin.c:95
    path=0x0, e=0x0, _ret=0x7ffe646452c0) at ../src/core/manager.c:1965
    name=0x5625ebb186f8 "local-fs.target", path=0x0, add_reference=true,
    mask=UNIT_DEPENDENCY_MOUNTINFO_IMPLICIT) at ../src/core/unit.c:2946
    where=0x5625ed9b3cc0 "/tmp", options=0x5625ed947110 "rw,nosuid,nodev,seclabel",
    fstype=0x5625ed95be90 "tmpfs", flags=0x7ffe64645395) at ../src/core/mount.c:1439
    where=0x5625ed9b3cc0 "/tmp", options=0x5625ed947110 "rw,nosuid,nodev,seclabel",
    fstype=0x5625ed95be90 "tmpfs", set_flags=false) at ../src/core/mount.c:1567
    at ../src/core/mount.c:1635
    ret_retval=0x7ffe64645660, ret_shutdown_verb=0x7ffe646456c0, ret_fds=0x7ffe646456d8,
    ret_switch_root_dir=0x7ffe646456b0, ret_switch_root_init=0x7ffe646456b8,
    ret_error_message=0x7ffe646456c8) at ../src/core/main.c:1669