meson: Rework clang-tidy integration to be done via unit tests

Instead of using run-clang-tidy.py with its own scheduling, let's
just gather a list of source files ourselves and then use that to
add a unit test for each source file that runs clang-tidy on the
source file.

We also add a bit of logic to run clang-tidy on most header files
as well for extra coverage. This uncovered various header files that
were not standalone so this commit also includes fixes to make sure
the clang-tidy tests are all green.

We can also use this in a later commit to run clang-include-cleaner
on each source file in the same way.

src/libsystemd-network/meson.build

@@ -1,6 +1,6 @@
 # SPDX-License-Identifier: LGPL-2.1-or-later
 
-sources = files(
+libsystemd_network_sources = files(
         'arp-util.c',
         'dhcp-network.c',
         'dhcp-option.c',
@@ -36,9 +36,11 @@ sources = files(
         'sd-radv.c',
 )
 
+sources += libsystemd_network_sources
+
 libsystemd_network = static_library(
         'systemd-network',
-        sources,
+        libsystemd_network_sources,
         include_directories : includes,
         implicit_include_directories : false,
         dependencies : userspace,