add pregenerated metadata cache for speed.

BUG=chromium-os:21049
TEST=emerge -s a pkg or two from this repo, checking
     /var/cache/edb/dep/ to ensure the cache isn't regenerated
     incorrectly

Change-Id: Ic15261732462fa319febf652a533722fcb0e2d4f
Reviewed-on: https://gerrit.chromium.org/gerrit/10254
Reviewed-by: David James <davidjames@chromium.org>
Reviewed-by: Chris Sosa <sosa@chromium.org>
Commit-Ready: Brian Harring <ferringb@chromium.org>
Tested-by: Brian Harring <ferringb@chromium.org>