core: change top-level drop-in from -.service.d to service.d

Discussed in #13743, the -.service semantic conflicts with the
existing root mount and slice names, making this feature not
uniformly extensible to all types. Change the name to be
<type>.d instead.

Updating to this format also extends the top-level dropin to
unit types.

test/TEST-15-DROPIN/test-dropin.sh

@@ -101,18 +101,18 @@ test_basic_dropins () {
     check_ok b Wants c.service
     systemctl stop a c
 
-    echo "*** test -.service.d/ top level drop-in"
+    echo "*** test service.d/ top level drop-in"
     create_services a b
     check_ko a ExecCondition "/bin/echo a"
     check_ko b ExecCondition "/bin/echo b"
-    mkdir -p /usr/lib/systemd/system/-.service.d
-    cat >/usr/lib/systemd/system/-.service.d/override.conf <<EOF
+    mkdir -p /usr/lib/systemd/system/service.d
+    cat >/usr/lib/systemd/system/service.d/override.conf <<EOF
 [Service]
 ExecCondition=/bin/echo %n
 EOF
     check_ok a ExecCondition "/bin/echo a"
     check_ok b ExecCondition "/bin/echo b"
-    rm -rf /usr/lib/systemd/system/-.service.d
+    rm -rf /usr/lib/systemd/system/service.d
 
     clear_services a b c
 }