Had the Go team made an assumption, they may have leveraged filesystem case-sensitiveness (or rather, case preservation†) to follow the same rule as for package

Had the Go team made an assumption, they may have leveraged filesystem case-sensitiveness (or rather, case preservation†) to follow the same rule as for package exports: capitalised => exported, lowercase first => not exported. The only assumption here is that the filesystem supports a tree. † This means any current filesystem except FAT which has (had?) a habit of changing the case of same-cap names in one way or the other. Cap-changing renames are a chore on some systems though.