build-system/asdf: Handle all asdf dependency specifications.

build-system/asdf: Handle all asdf dependency specifications.

Add support for dependencies of the form (:version <name> <version>),
(:feature <feature> <dependency-specification>) and (:require <module-name>),
as defined by
<https://common-lisp.net/project/asdf/asdf.html#The-defsystem-grammar>.

* guix/build/lisp-utils.scm (normalize-dependency): New variable.
(make-asd-file)[dependencies]: Use it to generate dependencies with normalized
names.
[dependency-name]: New variable.
[registry]: Use it to flatten the normalized dependencies.

Signed-off-by: Ludovic Courtès <ludo@gnu.org>