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.

* guix/packages.scm (%graft?, set-grafting): Move to...
* guix/derivations.scm: ... here.

* guix/gexp.scm (<gexp-compiler>): New record type.
  (%gexp-compilers): New variable.
  (register-compiler!, lookup-compiler): New procedures.
  (define-gexp-compiler): New macro.
  (origin-compiler, package-compiler): New compilers.
  (lower-inputs): Remove clauses for 'origin?' and 'package?'.  Add
  clause with 'lookup-compiler' instead.
  (lower-references): Likewise.
  (gexp-inputs)[add-reference-inputs]: Likewise.
  (gexp->sexp)[reference->sexp]: Likewise.

* guix/gexp.scm (gexp-inputs)[add-reference-inputs]: Remove clause for
  inputs of the form (PACKAGE OUTPUT).
  (gexp->sexp)[reference->sexp]: Likewise.
* tests/gexp.scm ("input list splicing"): Change 'list' to 'gexp-input'
  for glibc:debug.
  ("text-file*"): Likewise for %bootstrap-guile:out.
  ("input list splicing + gexp-input + ungexp-native-splicing"): Remove,
  now redundant.

* guix/profiles.scm (package->manifest-entry): Use 'gexp-input' instead
  of two-element lists to denote specific package outputs.
  (manifest-inputs): Likewise.
  (profile-derivation)[info-dir]: Likewise.

* guix/gexp.scm (<gexp-input>)[gexp-input]: Rename to...
  [%gexp-input]: ... this.  Adjust callers accordingly.
  (gexp-input): New procedure.
  (gexp-inputs)[add-reference-inputs]: When the input is a list, check
  whether each item is already 'gexp-input?' and to not rewrap those.
  (gexp-outputs)[add-reference-output]: Likewise.
  (gexp->sexp): Likewise.
* tests/gexp.scm ("input list splicing + gexp-input +
  ungexp-native-splicing"): New test.

* guix/gexp.scm (<gexp-input>): New record type.
  (gexp-inputs)[add-reference-inputs]: Adjust clauses to expect
  <gexp-input> objects.
  (gexp-outputs)[add-reference-output]: Likewise.
  (gexp->sexp)[reference->sexp]: Likewise.
  (canonicalize-reference): Remove.
  (gexp)[escape->ref]: Use 'gexp-input' for all the references.
  Remove use of 'canonicalize-reference'.

* guix/gexp.scm (<output-ref>): Rename to...
  (<gexp-output>): ... this.  Adjust constructor/accessor names and
  users accordingly.

* tests/gexp.scm (gexp-inputs, gexp-native-inputs, gexp-outputs,
  gexp->sexp): Make an indirection, to facilitate live testing with
  Geiser.

* gnu/packages/databases.scm (perl-dbix-class-schema-loader): New variable.

* gnu/packages/language.scm (perl-string-toidentifier-en): New variable.

* gnu/packages/perl.scm (perl-text-unidecode): New variable.

* gnu/packages/perl.scm (perl-string-camelcase): New variable.

* gnu/packages/language.scm (perl-lingua-en-inflect-phrase): New variable.

* gnu/packages/language.scm (perl-lingua-en-tagger): New variable.

* gnu/packages/language.scm (perl-lingua-stem): New variable.

* gnu/packages/language.scm (perl-text-german): New variable.

* gnu/packages/language.scm (perl-snowball-swedish): New variable.

* gnu/packages/language.scm (perl-snowball-norwegian): New variable.

* gnu/packages/language.scm (perl-lingua-stem-snowball-da): New
  variable.

* gnu/packages/language.scm (perl-lingua-stem-ru): New variable.

* gnu/packages/language.scm (perl-lingua-stem-it): New variable.

* gnu/packages/language.scm (perl-lingua-stem-fr): New variable.

* gnu/packages/language.scm (perl-lingua-pt-stemmer): New variable.

* gnu/packages/perl.scm (perl-memoize-expirelru): New variable.

* gnu/packages/language.scm (perl-lingua-en-number-isordinal): New
  variable.

* gnu/packages/language.scm (perl-lingua-en-inflect-number): New
  variable.

* gnu/packages/language.scm (perl-lingua-en-inflect): New variable.

* gnu/packages/language.scm (perl-lingua-en-findnumber): New variable.

* gnu/packages/language.scm: New file.
* gnu-system.am (GNU_SYSTEM_MODULES): Add it.

* gnu/packages/perl.scm (perl-class-unload): New variable.

* gnu/packages/perl.scm (perl-test-pod): New variable.

* gnu/packages/perl.scm (perl-moosex-nonmoose): New variable.

* gnu/packages/perl.scm (perl-moosex-markasmethods): New variable.

* gnu/packages/databases.scm (perl-dbix-class-introspectablem2m): New
  variable.

* gnu/packages/perl.scm (perl-config-general): New variable.

* gnu/packages/graphics.scm (brdf-explorer): New variable.

* guix/licenses.scm (ms-pl): New variable.

commit was still missing gnu-system.am sections and added patches

* gnu/packages/dictionaries.scm (sdcv): New variable.