Generate models into separate `models` package

[steebchen]

When querying for data, we already use the uppercase model name such as User for the query builder. This means we can't use the same name from the same package as a return type (e.g. of a FindOne call), which is why these types are currently suffixed with Model, such as UserModel. We could move those to a models package inside of whatever output folder is specified for the go client.

So, instead of:

func (x X) getUser() (prisma.UserModel, error) {
  return x.prisma.User.FindOne(
    db.User.ID.Equals("hi"),
  ).Exec(ctx)
}

the code could look like:

func (x X) getUser() (models.User, error) { // models: return type
  return x.prisma.User.FindOne(
    db.User.ID.Equals("hi"), // db: query builder etc.
  ).Exec(ctx)
}

[tarrencev]

I like this. More concise api