[dplug-build] Option to copy output in current directory

Most scripting need a way to get to the plugin binary without too much hassle. Add a switch --here that copies plugin binary right in the current directory after a build; much like --publish does

Start by locating dplug-build's existing --publish option and inspect how it handles the post-build plugin binary. Add the analogous --here behavior so the binary is copied into the current directory, then verify the output from a build using the new switch.

Written by the indexing model from the issue text.