core: reload cache if it's dirty when starting a UNIT_NOT_FOUND unit

The time-based cache allows starting a new unit without an expensive
daemon-reload, unless there was already a reference to it because of
a dependency or ordering from another unit.
If the cache is out of date, check again if we can load the
fragment.

test/units/testsuite-48.service

@@ -0,0 +1,7 @@
+[Unit]
+Description=TEST-48-START-STOP-NO-RELOAD
+
+[Service]
+ExecStartPre=rm -f /failed /testok
+ExecStart=/usr/lib/systemd/tests/testdata/units/%N.sh
+Type=oneshot

test/units/testsuite-48.sh

@@ -0,0 +1,38 @@
+#!/usr/bin/env bash
+# -*- mode: shell-script; indent-tabs-mode: nil; sh-basic-offset: 4; -*-
+# ex: ts=8 sw=4 sts=4 et filetype=sh
+set -ex
+
+cat > /run/systemd/system/testservice-48.target <<EOF
+[Unit]
+Wants=testservice-48.service
+EOF
+
+systemctl daemon-reload
+
+systemctl start testservice-48.target
+
+# The filesystem on the test image, despite being ext4, seems to have a mtime
+# granularity of one second, which means the manager's unit cache won't be
+# marked as dirty when writing the unit file, unless we wait at least a full
+# second after the previous daemon-reload.
+# May 07 23:12:20 systemd-testsuite testsuite-48.sh[30]: + cat
+# May 07 23:12:20 systemd-testsuite testsuite-48.sh[30]: + ls -l --full-time /etc/systemd/system/testservice-48.service
+# May 07 23:12:20 systemd-testsuite testsuite-48.sh[52]: -rw-r--r-- 1 root root 50 2020-05-07 23:12:20.000000000 +0100 /
+# May 07 23:12:20 systemd-testsuite testsuite-48.sh[30]: + stat -f --format=%t /etc/systemd/system/testservice-48.servic
+# May 07 23:12:20 systemd-testsuite testsuite-48.sh[53]: ef53
+sleep 1.1
+
+cat > /run/systemd/system/testservice-48.service <<EOF
+[Service]
+ExecStart=/bin/sleep infinity
+Type=exec
+EOF
+
+systemctl start testservice-48.service
+
+systemctl is-active testservice-48.service
+
+echo OK > /testok
+
+exit 0