Automatically get new eclass from upstream during package upgrade.
Automatically get new eclass from upstream during package upgrade.

When a newly upgraded package requies an eclass that our source does
not have the cros_portage_upgrade script will now automatically copy
that eclass from upstream into the same repository as the upgraded
package.

BUG=chromium-os:20763
TEST=Upgraded eselect package, which requires new bash-completion-r1 eclass.
Also upgraded eselect-ctag package, which requires no eclass.  Still works.

Change-Id: I7c207d9c4e3bea241081620deadd67e01caaedee
Reviewed-on: http://gerrit.chromium.org/gerrit/8210
Reviewed-by: David James <f231dc574bbeb62604660cbc162d1b8642a0a3af@chromium.org>
Commit-Ready: Matt Tennant <46e612415b446ccde19638e4035892001c237942@chromium.org>
Tested-by: Matt Tennant <46e612415b446ccde19638e4035892001c237942@chromium.org>

coreos/chromite,chadversary/chromiumos.chromite,zhang0137/chromite,zhang0137/chromite,chadversary/chromiumos.chromite,coreos/chromite,bpsinc-native/src_third_party_chromite,bpsinc-native/src_third_party_chromite,coreos/chromite,zhang0137/chromite,bpsinc-native/src_third_party_chromite