refactor: IWYU applied to the source files

`alternative_namespaces` examples left out as I consider them a subject for removal in the nearest future.

test/unit_test/static/custom_rep_test_min_impl.cpp

@@ -20,11 +20,9 @@
 // OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
 // SOFTWARE.
 
-#include "units/math.h"
-#include "units/isq/si/length.h"
-#include <chrono>
+#include <units/generic/dimensionless.h>
+#include <units/isq/si/length.h>
 #include <type_traits>
-#include <utility>
 
 namespace {