Add (guix channels) and use it in (guix scripts pull).

Add (guix channels) and use it in (guix scripts pull).

* guix/channels.scm: New file.
* Makefile.am (MODULES): Add it.
* guix/scripts/pull.scm: Use it.
(%default-options): Remove 'repository-url' and 'ref'.
(show-help, %options): Add '--channels'.
(%self-build-file, %pull-version, build-from-source)
(whole-package-for-legacy, derivation->manifest-entry): Remove.  These
now exist in a similar form in (guix channels).
(build-and-install): Change 'source' to 'instances'.  Remove #:url,
 #:branch, and #:commit.  Rewrite using 'channel-instances->manifest'.
(channel-list): New procedure.
(guix-pull): Parameterize %REPOSITORY-CACHE-DIRECTORY.  Call
'honor-lets-encrypt-certificates!' unconditionally.  Load
~/.config/guix/channels.scm.  Rewrite to use (guix channels).
[use-le-certs?]: Remove.
* po/guix/POTFILES.in: Add (guix channels).
* doc/guix.texi (Invoking guix pull): Group the description of '--url',
'--commit', and '--branch'.  Remove mention of 'GUIX_PULL_URL'.  Add
references to "Channels".  Document '--channels'.
(Channels): New node.
(Defining Packages): Link to "Channels" instead of "Package Modules".
(Invoking guix edit): Link to "Package Modules" instead of "Defining
Packages".
(Package Modules): Document both GUIX_PACKAGE_PATH and channels.