Turns out it's easier to read.
* tests/cpan.scm ("cpan->guix-package"): Use a quasiquoted pattern.
* tests/elpa.scm (eval-test-with-elpa): Likewise.
* tests/gem.scm ("gem->guix-package")
("gem->guix-package with a specific version")
("gem-recursive-import")
("gem-recursive-import with a specific version"): Likewise.
* tests/hexpm.scm ("hexpm-recursive-import"): Likewise.
* tests/opam.scm ("opam->guix-package"): Likewise.
* tests/pypi.scm ("pypi->guix-package, no wheel")
("pypi->guix-package, wheels")
("pypi->guix-package, no usable requirement file.")
("pypi->guix-package, package name contains \"-\" followed by digits"):
Likewise.
* tests/texlive.scm ("texlive->guix-package"): Likewise.
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.
