do not overwrite gokart.build namespace

[Hi-king]

https://github.com/m3dev/gokart/blob/489e2af926074d4c702c1c857b52834f664c1e81/gokart/init.py#L1

This line overwrites gokart.build module with gokart.build() function. We can't use gokart.build.GokartBuildError directly.

gokart.build module might ought to be renamed.