hex.pm is a package repository for Erlang and Elixir.
* guix/scripts/import.scm (importers): Add "hexpm".
* guix/scripts/import/hexpm.scm, guix/import/hexpm.scm,
guix/hexpm-download.scm: New files.
* guix/import/utils.scm (source-spec->object): Add "hexpm-fetch" to list of
fetch methods.
* guix/upstream.scm (package-update/hexpm-fetch): New function.
(%method-updates) Add it.
* Makefile.am: Add them.
