Make systemd-network-generator co-owned by -udev and -networkd

... (rhbz#2328723)

The files systemd-networkd-generator generates are read by udev (.link files)
and by networkd (.netdev, .netdev files). We can't move it to systemd-networkd
subpackage only, because that would potentially break the corner case of people
having systemd-udev installed and using the generator, but not systemd-networkd.
And there is no dependency from systemd-networkd to systemd-udev. I think this
is correct, because networkd can be used in containers without udev. But the
generator is not useful without either of those two daemons, so let's move
it to make the core package a bit lighter.

split-files.py

@@ -154,6 +154,9 @@ for file in files(buildroot):
           and os.path.exists(f'./{n}.example')):
         o = outputs['networkd-defaults']
 
+    # Files that are "consumed" by systemd-networkd go into the -networkd
+    # subpackage. As a special case, network-generator is co-owned also by
+    # the -udev subpackage because systemd-udevd reads .link files.
     elif re.search(r'''/usr/lib/systemd/network/.*\.network|
                        networkd|
                        networkctl|
@@ -164,6 +167,8 @@ for file in files(buildroot):
                        systemd\.netdev
     ''', n, re.X):
         o = outputs['networkd']
+    elif 'network-generator' in n:
+        o = (outputs['networkd'], outputs['udev'])
 
     elif '.so.' in n:
         o = outputs['libs']
@@ -255,7 +260,10 @@ for file in files(buildroot):
 
     suffix = '*' if '/man/' in n else ''
 
-    print(f'{prefix}{n}{suffix}', file=o)
+    if not isinstance(o, tuple):
+        o = (o,)
+    for file in o:
+        print(f'{prefix}{n}{suffix}', file=file)
 
 if [print(f'ERROR: no file names were written to {o.name}')
     for name, o in outputs.items()