units: skip modprobe@.service if the unit appears to be already loaded

Possible alternative to #14819.

For me, setting RemainAfterExit=yes would be OK, but if people think that it
might cause issues, then this could be a reasonable alternative that still
let's us skip the invocation of the separate binary.

units/modprobe@.service

@@ -13,6 +13,7 @@ DefaultDependencies=no
 Before=sysinit.target
 Documentation=man:modprobe(8)
 ConditionCapability=CAP_SYS_MODULE
+ConditionPathExists=!/sys/module/%I
 
 [Service]
 Type=oneshot