preupdate_hook: move convertAssign code to preupdate_hook file

[charlievieth]

Move the convertAssign logic from convert.go to
sqlite3_opt_preupdate_hook.go since that is the
only place it is used.

[dolmen]

What about just adding the build tags to convert.go?

//go:build sqlite_preupdate_hook
// +build sqlite_preupdate_hook

[charlievieth]

What about just adding the build tags to convert.go?

//go:build sqlite_preupdate_hook
// +build sqlite_preupdate_hook

That's also a valid solution. My motivation for moving it to sqlite3_opt_preupdate_hook.go is that it is a relatively small file, it's nice to consolidate logic, and most of the file names match their respective build tags so I felt that it would be clearer / more consistent to move it there.