Hello Guix,

This patch is intended mostly for discussion.  It was brought up again
on IRC yesterday the idea of prefetching package source into the store
in order to be able to hack on guix offline.

A naive approach such as running `guix build -S` on each entry of `guix
package --list-available` isn't enough because of build-system-implicit
inputs and package origin inputs, as well as non-public bootstrapping
source.

This patch changes the semantics of `guix build --source` so that it
takes an optional argument, one of 'package', 'all' (default), or
'transitive'.  `guix build --source=package foo` will build the source
derivations for foo's source origin, which is the current behavior.
`guix build --source=all foo` will build all of foo's origin's,
including any which might be in its inputs (e.g. texlive's
texmf-extra-src input).  `guix build --source=transitive foo` will build
the source origins for all of foo's transitive inputs.