guix package: Refactor 'options->installable'.

* guix/scripts/package.scm (options->upgrade-predicate)
(store-item->manifest-entry): New procedures.
* guix/scripts/package.scm (options->installable): Use them.  Remove the
'packages-to-upgrade' and 'packages-to-install' variables by getting rid
of a level of indirection.

guix package: Move 'process-actions' out of sight.

* guix/scripts/package.scm (process-actions): New procedure, moved
from...
(guix-package): ... here.  Adjust accordingly.

guix package: Formalize the list of actions.

* guix/scripts/package.scm (roll-back-action, switch-generation-action)
(delete-generations-action, manifest-action): New procedures.
(%actions): New variable.
* guix/scripts/package.scm (guix-package)[process-action]: Rewrite to
traverse %ACTIONS.

guix package: Move 'build-and-use-profile' out of sight.

* guix/scripts/package.scm (build-and-use-profile): New procedure.
Adapted and moved from...
(guix-package)[process-actions]: ... here.  Adjust call sites.

guix package: Move a couple of procedures out of sight.

* guix/scripts/package.scm (ensure-default-profile, process-query): New
procedures, moved from...
(guix-package): ... here.

guix package: Remove unnecessary use of (%store).

* guix/scripts/package.scm (delete-matching-generations): Use STORE
instead of (%store).

guix package: Reduce startup time by ~50%.

As measured with:

  time sh -c 'for i in `seq 1 10` ; do guix package --search-paths ; done'

On my machine, when running:

  strace -o ,,s guix package --search-paths

the number returned by:

  grep -E '^(open|l?stat).*\.go' ,,s | wc -l

drops from 1610 to 837.

* guix/scripts/package.scm: Remove two unnecessary #:use-module forms.
Autoload (gnu packages ...) modules.

guix package: '--search-paths' can report combined search paths.

Partly fixes <http://bugs.gnu.org/20255>.

* guix/scripts/package.scm (search-path-environment-variables): Change
  'profile' to 'profiles'; expect it to be a list.
  (display-search-paths): Likewise.
  (%default-options): Remove 'profile' entry.
  (%options) <--profile>: Keep previous values associated with 'profile'
  in RESULT.
  (guix-package)[process-actions, process-query]: Handle the possible
  lack of 'profile' pair in OPTS.

Merge branch 'master' into dbus-update

ui: Add 'make-regexp*'.

Fixes <http://bugs.gnu.org/21773>.
Reported by Jan Synáček <jan.synacek@gmail.com>.

* guix/ui.scm (make-regexp*): New procedure.
* guix/scripts/package.scm (options->installable, guix-package): Use it
  when processing user-provided regexps.

profiles: Add generation manipulation procedures.

* guix/scripts/package.scm (delete-generations): Use
  'delete-generation*' instead of 'delete-generation'.
  (guix-package)[process-actions]: Use 'roll-back*' instead of
  'roll-back' and 'switch-to-generation*' instead of
  'switch-to-generation'.
  (link-to-empty-profile, switch-to-generation,
  switch-to-previous-generation, roll-back, delete-generation): Move
  to...
* guix/profiles.scm: ... here.  Adjust to not print messages and to
  return values that can be used by user interfaces.
* guix/ui.scm (display-generation-change, roll-back*,
  switch-to-generation*, delete-generation*): New procedures.

ui: Add procedures to display a profile generation.

* guix/scripts/package.scm (guix-package)[process-query](list-generations):
  Move part of the body to 'delete-generation' and
  'display-profile-content'.
* guix/ui.scm (display-generation, display-profile-content): New
  procedures.

utils: Add 'readlink*'.

* guix/scripts/package.scm (readlink*): Move to...
* guix/utils.scm (readlink*): ... here.  New procedure.

ui: Add 'matching-generations'.

* guix/scripts/package.scm (matching-generations): Move to...
* guix/ui.scm (matching-generations): ... here.

Merge branch 'master' into core-updates

Add (guix scripts).

* guix/ui.scm: Add missing copyright lines.
  (args-fold*, environment-build-options, %default-argument-handler,
  parse-command-line): Move to ...
* guix/scripts.scm: ...here.  New file.
* guix/scripts/archive.scm: Use it.
* guix/scripts/build.scm: Likewise.
* guix/scripts/download.scm: Likewise.
* guix/scripts/edit.scm: Likewise.
* guix/scripts/environment.scm: Likewise.
* guix/scripts/gc.scm: Likewise.
* guix/scripts/graph.scm: Likewise.
* guix/scripts/hash.scm: Likewise.
* guix/scripts/import/cpan.scm: Likewise.
* guix/scripts/import/cran.scm: Likewise.
* guix/scripts/import/elpa.scm: Likewise.
* guix/scripts/import/gem.scm: Likewise.
* guix/scripts/import/gnu.scm: Likewise.
* guix/scripts/import/hackage.scm: Likewise.
* guix/scripts/import/nix.scm: Likewise.
* guix/scripts/import/pypi.scm: Likewise.
* guix/scripts/lint.scm: Likewise.
* guix/scripts/package.scm: Likewise.
* guix/scripts/publish.scm: Likewise.
* guix/scripts/pull.scm: Likewise.
* guix/scripts/refresh.scm: Likewise.
* guix/scripts/size.scm: Likewise.
* guix/scripts/system.scm: Likewise.
* tests/ui.scm (with-environment-variable, "parse-command-line",
  "parse-command-line and --no options"): Move to ...
* tests/scripts.scm: ...here.  New file.
* Makefile.am (MODULES): Add guix/scripts.scm.
  (SCM_TESTS): Add tests/scripts.scm.
* po/guix/POTFILES.in: Add guix/scripts.scm.

scripts: package: Add --install-from-file option.

* guix/scripts/package.scm (show-help): Add help text for --install-from-file
  option.
  (%options): Add --install-from-file option.
* tests/guix-package.sh: Test it.
* doc/guix.texi ("invoking guix package"): Document it.
* doc/package-hello.scm: New file.
* doc.am (EXTRA_DIST): Add it.

Merge branch 'master' into core-updates

ui: Add 'leave-on-EPIPE'.

* guix/scripts/package.scm (leave-on-EPIPE): Move to...
* guix/ui.scm (leave-on-EPIPE): ... here.

Merge branch 'master' into core-updates