packages: Add the channel .go files to the search path.

packages: Add the channel .go files to the search path.

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.