It is not needed to expand ~ in file completion as you type after
62c3aeb9c made commands themselves interpret it. We only need to call
expand-path on the directory before passing it to utils.readdir in order
to find files in it when it contains ~ placeholders. As a bonus this
will now also complete files in directories like ~~/ and ~state/.
This simplifies command completion, and was also the blocker for
splitting running commands out of console.lua, since I didn't know how
to replicate it through mp.input.