rename 'apt-get files' to 'apt-get indextargets'

rename 'apt-get files' to 'apt-get indextargets'

'files' is a bit too generic as a name for a command usually only used
programmatically (if at all) by developers, so instead of "wasting" this
generic name for this we use "indextargets" which is actually the name
of the datastructure the displayed data is stored in.
Along with this rename the config options are renamed accordingly.