ListUnitFiles impacts systemd a lot, causing high CPU consumption and delay in operations

Description of problem:

This is a respin of BZ #1828759 ("systemctl list-unit-files" kills the system usability).

With RHEL8.7 / systemd-239-68.el8 (didn't check with older ones yet), we can see that the ListUnitFiles DBus operation is very costly.
When executing systemd under strace, we can observe that on a system having 145 units ("minimal installation" of RHEL8.7), the readlinkat/openat/getdents64 syscalls are called hundreds or even thousands of times on same files (see reproducer below), causing high CPU consumption.

Version-Release number of selected component (if applicable):

systemd-239-68.el8

How reproducible:

Always

Steps to Reproduce:
1. Execute the DBus ListUnitFiles operation while stracing systemd

1. strace -CttTvyy -s 128 -o list-unit-files.strace -p 1 &
2. time dbus-send --system --print-reply --reply-timeout=20000 --type=method_call --dest=org.freedesktop.systemd1 /org/freedesktop/systemd1 org.freedesktop.systemd1.Manager.ListUnitFiles
3. kill %1

2. Check number of syscalls

1. tail -20 list-unit-files.strace
   % time seconds usecs/call calls errors syscall
   • • • • • ----------- ----------- --------- --------- ----------------
             30.08 0.277966 2 119048 readlinkat
             21.62 0.199787 2 79554 16869 openat
             21.01 0.194170 2 89605 getdents64
             10.02 0.092602 1 90426 fcntl
             ...

3. Check number of times readlinkat() is executed on a given path

1. grep " readlinkat(" list-unit-files.strace | cut -f2 -d'"' | sort | uniq -c | sort -k1 -rn > list-unit-files.strace.readlinkat
2. head list-unit-files.strace.readlinkat
   820 /etc/systemd/system/syslog.service
   819 /etc/systemd/system/multi-user.target.wants/remote-fs.target
   818 /etc/systemd/system/multi-user.target.wants/crond.service
   817 /etc/systemd/system/multi-user.target.wants/NetworkManager.service
   815 /etc/systemd/system/multi-user.target.wants/dnf-makecache.timer
   814 /etc/systemd/system/multi-user.target.wants/sssd.service
   ...

1. grep "NetworkManager.service" list-unit-files.strace.readlinkat
   817 /etc/systemd/system/multi-user.target.wants/NetworkManager.service
   793 /etc/systemd/system/dbus-org.freedesktop.NetworkManager.service

Actual results:

Slowness, CPU consumption

Expected results:

Files/Symlinks resolved once.