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/core/meson.build

@@ -78,6 +78,8 @@ if conf.get('BPF_FRAMEWORK') == 1
                 restrict_ifaces_skel_h]
 endif
 
+sources += libcore_sources
+
 load_fragment_gperf_gperf = custom_target(
         'load-fragment-gperf.gperf',
         input : 'load-fragment-gperf.gperf.in',

src/core/show-status.h

@@ -1,6 +1,7 @@
 /* SPDX-License-Identifier: LGPL-2.1-or-later */
 #pragma once
 
+#include <errno.h>
 #include <stdarg.h>
 #include <stdbool.h>