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/basic/meson.build

@@ -118,6 +118,8 @@ basic_sources = files(
         'xattr-util.c',
 )
 
+sources += basic_sources
+
 missing_audit_h = files('missing_audit.h')
 missing_socket_h = files('missing_socket.h')