How to write own ISR handler
The interrupt.h is not a library but a standard avr-gcc header file which provides a standard way of defining interrupt handlers to make the compiler know the function you are defining is an interrupt.
What you are suggesting to work without the header, while possible, makes little sense. It is similar to not wanting to use PORTB name for writing an IO port, but using the address of the IO port instead of the name, or not including any other standard library if it contains the functions you need.