Replace "copy" plugin by "dump" with std conventions

[Gustavo Niemeyer]

We've discussed this before a few times, but it probably got lost in the volume of good work going on.

The "copy" plugin is a bit of an alien at the moment, with its "files" parameter which takes a map of origin and destination and remains out of the standard organization and file selection which is available to every other plugin: organize map, stage list, snap list, file sets, etc.

It sounds like addressing this might be done elegantly by obsoleting the "copy" plugin and introducing a new "dump" plugin which simply unpacks the _whole_ tarball into the plugin installdir, and allows the standard features to be applied to it (organize, etc).

I've been using locally such a dump plugin which consists solely of:

    def build(self):
        super().build()
        self.run(['cp', '-a', '.', self.installdir])

[Sergio Schvezov (sergiusens)]

https://github.com/snapcore/snapcraft/pull/712