Until now the caller couldn't tell the different between a spec like
"foo:out" and one like "foo".  This change allows users to distinguish
between these two cases.

* gnu/packages.scm (specification->package+output): Disable output
membership test when OUTPUT = #f and SUB-DRV = #f.
* tests/packages.scm ("specification->package+output")
("specification->package+output invalid output")
("specification->package+output no default output")
("specification->package+output invalid output, no default"): New tests.

This is a followup to e2a903c807ccacec5925f197ce26f626060e1953.

* gnu/packages.scm (find-newest-available-packages): New procedure.

Until now %LOAD-COMPILED-PATH would wrongfully contain:

  CHANNEL/share/guile/site/X.Y

for each channel, thereby ignoring all the .go files of channels.  This
fixes it so that %LOAD-COMPILED-PATH instead contains:

  CHANNEL/lib/guile/X.Y/site-ccache

* guix/describe.scm (current-channel-entries): New procedure.
(package-path-entries): Change to return the %LOAD-COMPILED-PATH entries
as a second value.
* gnu/packages.scm (%package-module-path): Expect two values from
'package-path-entries' and augment %LOAD-COMPILED-PATH accordingly.

Previously the same package could appear several times if several
variables were bound to it, as is notably the case for "python"
currently.  This, in turn, would lead to obnoxious "ambiguous package
specification" messages.

* gnu/packages.scm (generate-package-cache)[expand-cache]: Change RESULT
to RESULT+SEEN and adjust accordingly.
Call 'first' on the result of 'fold-module-public-variables*'.
* tests/packages.scm ("fold-available-packages with/without cache"):
Check for lack of duplicates in FROM-CACHE.

* gnu/packages.scm (fold-available-packages): New procedure.
* guix/scripts/package.scm (process-query): Use it instead of
'fold-packages'.
* tests/packages.scm ("fold-available-packages with/without cache"):
New test.

That way 'guix edit' doesn't need to load any package module.

* gnu/packages.scm (find-package-locations, specification->location):
New procedures.
* guix/scripts/edit.scm (package->location-specification): Rename to...
(location->location-specification): ... this.  Expect a location object
instead of a package.
(guix-edit): Use 'specification->location' instead of
'specification->package'.
* tests/packages.scm ("find-package-locations")
("find-package-locations with cache")
("specification->location"): New tests.

* gnu/packages.scm (cache-is-authoritative?, load-package-cache)
(cache-lookup, generate-package-cache): New procedures.
(%package-cache-file): New variable.
(find-packages-by-name): Rename to...
(find-packages-by-name/direct): ... this.
(find-packages-by-name): Rewrite to use the package cache when
'cache-is-authoritative?' returns true.
* tests/packages.scm ("find-packages-by-name + version, with cache")
("find-packages-by-name with cache"): New tests.
* guix/channels.scm (package-cache-file): New procedure.
(%channel-profile-hooks): New variable.
(channel-instances->derivation): Use it in #:hooks.
* guix/scripts/package.scm (build-and-use-profile): Add #:hooks and
honor it.
* guix/scripts/pull.scm (build-and-install): Pass #:hooks to
UPDATE-PROFILE.

Since commit 9ffc1c00e55eb7931846dbb3fafcf54716fff57c,
'find-newest-available-packages' and 'find-packages-by-name' were both
building a vhash mapping package names to packages.  This factorizes
this bit, also reducing I/O, CPU, and memory usage.

* gnu/packages.scm (find-best-packages-by-name): Remove.
(find-best-packages-by-name): Use 'find-packages-by-name' instead of
'find-newest-available-packages'.

* guix/describe.scm: New file.
* Makefile.am (MODULES): Add it.
* gnu/packages.scm (%default-package-module-path): New variable.
(%package-module-path): Honor 'package-path-entries'.
* build-aux/update-NEWS.scm (main): Use %DEFAULT-PACKAGE-MODULE-PATH
instead of (last (%package-module-path)).

Fixes <https://bugs.gnu.org/28446>.
Reported by Mark H Weaver <mhw@netris.org>.

* gnu/packages.scm (find-packages-by-name): Use 'version-prefix?'
instead of 'string-prefix?'.