PACCAR has reported higher sales and net income for the second quarter and first half of 2002 compared with the same periods a year ago. The company said that orders are surging in the US heavy duty truck market ahead of more costly engines due later this year.   Second quarter net sales and financial services revenues were $1.8 billion, 18 percent higher than the $1.5 billion reported for the comparable period in 2001. Net income of $73.7 million ($.63 per diluted share) increased 87 percent from the $39.5 million ($.34 per diluted share) earned in the second quarter of 2001.