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/meson.build

@@ -137,6 +137,8 @@ libsystemd_sources = files(
   + sd_login_sources + sd_json_sources + sd_varlink_sources \
   + sd_path_sources + sd_netlink_sources + sd_network_sources
 
+sources += libsystemd_sources
+
 libsystemd_c_args = ['-fvisibility=default']
 
 libsystemd_static = static_library(