Internal: do not rely on re-exports

Internal modules explicitly import third-party
dependencies rather than relying on re-exports
from MPD.Core.*
Move run{,with} to MPD.Core.Run
MPD.Core: remove third-party re-exports
cabal: move internal modules to other-modules
Haddock: more internal links
Commands.ParserSpec: generate arbitrary SongInfo
Implement ADT for PlaybackState
Commands: implement ADT for subsystem names
Implement wrapper for seek
Implement wrapper for "seekcur"