From now own, (guix packages) depends on (guix gexps); it was the other
way around now. This means that (guix packages) code can use gexps.
* guix/gexp.scm (origin-compiler, package-compiler): Remove.
(default-guile-derivation): New procedure.
(gexp->derivation): Use it instead of 'default-guile' +
'package->derivation'.
* guix/packages.scm (default-guile-derivation): New procedure.
(package-compiler, origin-compiler): New variables.
* doc/guix.texi (G-Expressions): Mention extensibility.
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