packages: Add missing export.

* guix/packages.scm: Export `package-home-page'.

Move base32 code to (guix base32).

* guix/utils.scm (bytevector-quintet-ref, bytevector-quintet-ref-right,
  bytevector-quintet-length, bytevector-quintet-fold,
  bytevector-quintet-fold-right, make-bytevector->base32-string,
  %nix-base32-chars, %rfc4648-base32-chars, bytevector->base32-string,
  bytevector->nix-base32-string, bytevector-quintet-set!,
  bytevector-quintet-set-right!, base32-string-unfold,
  base32-string-unfold-right, make-base32-string->bytevector,
  base32-string->bytevector, nix-base32-string->bytevector): Move to...
* guix/base32.scm: ... here.  New file.

* tests/utils.scm (%nix-hash, "bytevector->base32-string",
  "base32-string->bytevector", "nix-base32-string->bytevector", "sha256
  & bytevector->base32-string"): Move to...
* tests/base32.scm: ... here.  New file

* guix-download.in, guix/derivations.scm, guix/packages.scm,
  guix/snix.scm, tests/builders.scm, tests/derivations.scm: Adjust
  accordingly.
* guix.scm (%public-modules): Add `base32'.

packages: `description' → `synopsis', `long-description' → `description'.

* guix/packages.scm (<package>): Rename `description' to `synopsis', and
  `long-description' to `description'.
* tests/packages.scm, distro/packages/base.scm,
  distro/packages/databases.scm, distro/packages/guile.scm,
  distro/packages/typesetting.scm:  Update accordingly.
* po/Makevars (XGETTEXT_OPTIONS): Update `--keyword' flags accordingly.

guix-build: Gracefully handle `&package-input-error' conditions.

* guix/packages.scm: Export `package-error?' and `package-input-error?'.
* guix-build.in (guix-build): Catch `&package-input-error' conditions,
  print a human-readable message, and exit.

packages: Pass `system' around.

* guix/packages.scm (package-source-derivation): Add `system'
  parameter.  Pass it to METHOD.
  (package-derivation)[expand-input]: Pass SYSTEM to
  `package-derivation' and `package-source-derivation'.

* distro/packages/base.scm (package-with-bootstrap-guile)[boot]: Pass
  SYSTEM to FETCH.

packages: Add support for system-dependent inputs.

* guix/packages.scm (package-derivation)[intern]: New procedure.  Pass
  #t as the `recursive?' argument, instead of #f.
  [expand-input]: New procedure, with code formerly in the body.
  Support inputs where the input is a procedure returning a file name or
  an <origin>.
  Use `expand-input' in the body.

* tests/packages.scm ("trivial with system-dependent input"): New test.

packages: Fix and optimize memoization of `package-derivation'.

* guix/packages.scm (%derivation-cache): Pass an initial size of 100.
  (cache): Use `hashq-set!', and use a SYSTEM/DRV pair as the value.
  (cached-derivation): Update accordingly.

packages: Micro-optimize `package-derivation'.

* guix/packages.scm (package-derivation): Move `cache' call before the
  traversal of PACKAGE's inputs.

Add (guix build-system trivial).

* guix/build-system/trivial.scm: New file.
* Makefile.am (MODULES): Add it.
* tests/packages.scm ("trivial"): New test.

* guix/packages.scm (package-derivation): Allow SOURCE to be #f.

packages: Accept <origin> as package inputs.

* guix/packages.scm (package-derivation): Accept use of an <origin> as
  an input.

packages: Add `package-full-name'.

* guix/packages.scm (package-full-name): New procedure.
  (package-derivation): Use it.

Move <location> to (guix utils).

* guix/packages.scm (<location>, location): Move to...
* guix/utils.scm: ... here.

packages: Add `package-transitive-propagated-inputs'.

* guix/packages.scm (transitive-inputs): New procedure.
  (package-transitive-inputs): Rewrite in terms of `transitive-inputs'.
  (package-transitive-propagated-inputs): New procedure.

packages: Cache the result of `package-derivation'.

* guix/packages.scm (%derivation-cache): New variable.
  (cache, cached-derivation): New procedures.
  (package-derivation): Use them.

packages: Add a printer for <package>.

* guix/packages.scm (<package>): Add record printer.

packages: Raise an error condition upon invalid input.

* guix/packages.scm (&package-error, &package-input-error): New
  condition types.
  (package-derivation): Raise a `&package-input-error' when no match is
  made.

packages: Add all non-derivation paths to the store.

* guix/packages.scm (package-derivation): Add all non-derivation paths
  to the store.

packages: Allow store paths as inputs.

* guix/packages.scm (package-derivation): Allow non-derivation store
  paths as inputs.

Add `package-transitive-inputs'; use it to honor propagated inputs.

* guix/packages.scm (package-transitive-inputs): New procedure.
  (package-derivation): Use it to compute INPUTS.

* tests/packages.scm (dummy-package): New macro.
  ("package-transitive-inputs"): New test.

packages: Allow inputs that are local files.

* guix/packages.scm (package-derivation): Add to the store inputs that
  are local files.