When mysql is built, it runs a tool called comp_err.
From MySQL's documentation:
comp_err creates the errmsg.sys file that is used by mysqld to determine the error messages to display for different error codes. comp_err normally is run automatically when MySQL is built. It compiles the errmsg.sys file from the text file located at sql/share/errmsg-utf8.txt in MySQL source distributions.
In essence, the internal package contains an extremely stripped down version of comp_err, which only handles the error definitions and error codes, as we need them, but ignores the translations and error messages.
The Stringer implementation is provided by the Go team's fantastic stringer tool.