mem: Rename the ruby Prefetcher class RubyPrefetcher.

mem: Rename the ruby Prefetcher class RubyPrefetcher.

A new Prefetcher namespace was added which holds the gem5 prefetchers
and means they don't all need a "Prefetcher" in their name. Unfortunately
that means that there is now both a Prefetcher namespace and a
Prefetcher class which conflict with each other.

This change tries to resolve the conflict with as little disruption as
possible by simply renaming the c++ ruby Pretcher class RubyPrefetcher,
leaving the python name alone so that configs aren't affected.

Issue-on: https://gem5.atlassian.net/browse/GEM5-447

Change-Id: I7afdf5dbc57dbf46d82552113c52f3a9207870f2
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/27949
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Maintainer: Gabe Black <gabeblack@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>