channels: Compute a package cache and use it.

channels: Compute a package cache and use it.

* 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.