It’s not surprising that you didn’t find it, as imports isn’t defined by gnome-shell at all, but by gjs itself.
Keep in mind that both gjs and gnome-shell predate ecmascript modules (i.e. import/export statements) in javascript, so gjs had to come up with its own import system.
(gjs does support standard modules nowadays, and the plan is to port to them eventually, hopefully in time for 43. But it’s a breaking change for extensions, so will need some care and communication)