This change allows ELPA packages to have their src attribute updated
by overrideAttrs. Without this change the installPhase references the
original src attribute and overriding is not possible.
Previously, meta wasn't being passed through at all, because it's
removed from args without being used anywhere. This made it so that
rcirc-menu wasn't being marked as broken even though it was supposed
to be.
This patch copies the meta handling from melpaBuild, including the
default home page (adapted for ELPA).
Building packages requires package-build.el from Melpa, but installing
packages only requires package.el. Packages from ELPA are already built,
so there is no need to involve package-build.el.
