cubicweb-dataio #3479739 Drop directory argument in ccplugin command [validation pending]

Currently, only one level of directory is handled and some files are ignored silently depending on their extensions. We should do it properly or nothing.

prioritynormal
typebug
done in0.4.0
load0.100
load left0.000
closed by#e649822268e9 [ccplugin] stop supporting directory as argument until it's properly documented, closes #3479739
patch[ccplugin] stop supporting directory as argument until it's properly documented [applied]