wrap the cover_names map with a list in fetchart
wrap the cover_names map with a list in fetchart

most `cover_names` users expect to operate on a list

SusannaMaria/beets,sampsyo/beets,SusannaMaria/beets,pkess/beets,jcoady9/beets,MyTunesFreeMusic/privacy-policy,madmouser1/beets,beetbox/beets,diego-plan9/beets,jackwilsdon/beets,beetbox/beets,xsteadfastx/beets,SusannaMaria/beets,pkess/beets,MyTunesFreeMusic/privacy-policy,madmouser1/beets,Kraymer/beets,artemutin/beets,sampsyo/beets,Kraymer/beets,Kraymer/beets,jackwilsdon/beets,MyTunesFreeMusic/privacy-policy,lengtche/beets,lengtche/beets,beetbox/beets,mosesfistos1/beetbox,diego-plan9/beets,jackwilsdon/beets,artemutin/beets,mosesfistos1/beetbox,SusannaMaria/beets,jackwilsdon/beets,shamangeorge/beets,jcoady9/beets,shamangeorge/beets,ibmibmibm/beets,lengtche/beets,ibmibmibm/beets,lengtche/beets,diego-plan9/beets,pkess/beets,mosesfistos1/beetbox,ibmibmibm/beets,madmouser1/beets,shamangeorge/beets,artemutin/beets,shamangeorge/beets,jcoady9/beets,xsteadfastx/beets,mosesfistos1/beetbox,xsteadfastx/beets,madmouser1/beets,diego-plan9/beets,Kraymer/beets,MyTunesFreeMusic/privacy-policy,ibmibmibm/beets,sampsyo/beets,artemutin/beets,jcoady9/beets,sampsyo/beets,pkess/beets,beetbox/beets,xsteadfastx/beets