pull: Build with the matching Guile major version.

pull: Build with the matching Guile major version.

Previously, 'guix pull' would always build with Guile 2.0.
Now it builds with the Guile that matches (effective-version).

* build-aux/build-self.scm (false-if-wrong-guile)
(package-for-current-guile): New procedures.
(guile-json, guile-ssh): Use it.
(guile-for-build): New procedure.
(build): Use (effective-version) instead of the hard-coded "/2.0".
Add (guix modules) closure to #:modules argument.  Pass
\#:guile-for-build argument to 'gexp->derivation'.
* guix/build/pull.scm (depends-on-guile-ssh?, all-scheme-files): New
procedures.
(build-guix): Show the output of (version).  Use the above procedures.
Filter out files that match 'depends-on-guile-ssh?' when (ssh session)
is missing.