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/runtime/fmt_test.cpp

@@ -20,14 +20,17 @@
 // OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
 // SOFTWARE.
 
+#include <units/customization_points.h>
 #include <units/format.h>
-#include <units/math.h>
-#include <units/isq/si/si.h>
+#include <units/generic/dimensionless.h>
 #include <units/isq/si/cgs/cgs.h>
+#include <units/isq/si/si.h>
+#include <units/math.h> // IWYU pragma: keep
 #include <units/quantity_io.h>
 #include <catch2/catch.hpp>
 #include <iomanip>
-#include <sstream>
+#include <limits>
+#include <locale>
 
 using namespace units;
 using namespace units::isq;