“What are we even talking about when we talk about mothers?” The question, posed by Regina (Sophie Okonedo) in the midst of a drug-hazy cuddle puddle, is the central idea that animates Annie Baker’s verdant, sun-dappled debut feature “Janet Planet.” For our heroine, 11-year-old Lacy (Zoe Ziegler, in her first film role), it is the East (1991 Northampton, Massachusetts to be exact) and Janet (Julianne Nicholson) is the sun. Lacy’s entire galaxy orbits around her mom, whose gravitational pull draws in a variety of suitors, lovers and friends, all impeding on Lacy’s time and attention from her mother.