Fixes <https://bugs.gnu.org/29091>.
Reported by Eric Bavier <bavier@cray.com>.
* guix/build/compile.scm (relative-file): New procedure.
(load-files): Use it before calling 'file-name->module-name'.
(compile-files): Likewise before calling 'scm->go'.
* guix/build/pull.scm (build-guix): Remove 'with-directory-excursion'
and file name hack from ce33c3af76b0e5c68cc42dddf2b9c4b017386fd8.
Pass OUT to 'all-scheme-files'.