chore(build): use third_party.go

use the third_party.go project to replace our update script. This
requires moving a few things around and gets rid of a few annoying bugs:

- You can now bump individual packages
- A new src directory isn't created on build
- Less shell scripting!
- Things get built into ./bin/

third_party/github.com/stretchr/testify/assert/errors.go

@@ -1,10 +0,0 @@
-package assert
-
-import (
-	"errors"
-)
-
-// AnError is an erorr instance useful for testing.  If the code does not care
-// about error specifics, and only needs to return the error for example, this
-// error should be used to make the test code more readable.
-var AnError error = errors.New("assert.AnError general error for testing.")