guix build: Transformations operate on single objects.

* guix/scripts/build.scm (transform-package-source): Return a procedure
that expects a single object rather than a list of packages.
(options->transformation): Rewrite to precompute the list of applicable
transformations and to return a procedure that expects a single object
rather than a list of objects.
(options->derivations): Adjust accordingly.
* tests/scripts-build.scm: New file.
* Makefile.am (SCM_TESTS): Add it.