guix-package: Extract version strings when installing a direct store path.

* guix-package.in (guix-package)[process-actions]: Extract the version
  string from store paths.
* tests/guix-package.sh: Adjust accordingly.

guix-package: Record the correct version string in manifests.

* guix-package.in (guix-package)[find-package]: Return (package-version p),
  not VERSION.

guix-package: Add `--list-available'.

* guix-package.in (show-help, %options): Add `--list-available'.
  (guix-package)[process-query]: Add support for `--list-available'.
* doc/guix.texi (Invoking guix-package): Document it.
* tests/guix-package.sh: Add test.

* guix/ui.scm (location->string): New procedure.
* guix/utils.scm: Export <location>.

guix-package: Add `--list-installed'.

* guix-package.in (show-help, %options): Add `--list-installed'.
  (guix-package): Move main body to...
  [process-actions]: ... here.  New internal procedure.
  [process-query]: New procedure.
* tests/guix-package.sh: Add tests for `--list-installed'.
* doc/guix.texi (Invoking guix-package): Document it.

guix-package: Fix typo.

* guix-package.in (guix-package)[find-package]: Don't use `_' as the
  wildcard, to avoid collision with `gettext'.

guix-package: Fix handling of the PACKAGE:OUTPUT syntax.

* guix-package.in (guix-package)[find-package]: Return the correct NAME
  and SUB-DRV when NAME contains #\:.

* tests/guix-package.sh (profile): Add test.

guix-package: Fix invalid module use.

* guix-package.in: Use (distro packages guile), not (... base).

guix-package: Remove extraneous procedures.

* guix-package.in (_, N_): Remove.

utils: Add `package-name->name+version'.

* guix/utils.scm (package-name->name+version): New procedure.
* guix-package.in (guix-package)[find-package]: Use it.
* tests/utils.scm ("package-name->name+version"): New test.

guix-package: Use more (guix ui) features.

* guix-package.in (leave): Remove.
  (guix-package): Wrap body in `with-error-handling'.

ui: Factorize `--version'.

* guix/ui.scm (show-version-and-exit): New procedure.
* guix-build.in (show-version): Remove.
  (%options)["version"]: Use `show-version-and-exit'.
* guix-download.in: Likewise.
* guix-package.in: Likewise.

Add a preliminary `guix-package' command-line tool.

* guix-package.in, tests/guix-package.sh: New files.
* configure.ac: Output `guix-package'.
* Makefile.am (TESTS): Add `tests/guix-package.sh'.
  (bin_SCRIPTS): Add `guix-package'.