* support list of repos for hex packages
repos are defined under the hex key in rebar configs. They can be
defined at the top level of a project or globally, but not in
profiles and the repos configured in dependencies are also ignored.
Searching for packages involves first checking for a match in the
local repo index cache, in the order repos are defined. If not found
each repo is checked through the hex api for any known versions of
the package and the first repo with a version that fits the constraint
is used.
* add {repos, replace, []} for overriding the global & default repos
Tristan Sloughter
Takeru Ohta