Britney Spearsis packing on the pounds, according to pals who claimed the “Oops! … I Did It Again” singer has tossed diets and healthy eating out the window in favor of comforting junk food to banish her blues. The 42-year-old singer was spotted out and about as she landed at Van Nuys airport following another Mexico vacation where she could wallow in savory treats and sweet drinks without fear of tattletale spies, RadarOnline.com has learned. “Food is one of the big comforts she has in life, whether she’s on holiday or at home,” dished an insider. “She loves Mexican food and those slushy co…