derivations: Import modules with 'add-file-tree-to-store'.

derivations: Import modules with 'add-file-tree-to-store'.

This reduces the number of RPCs on "guix build libreoffice -nd" from
2,589 to 2,558 (1%).

* guix/derivations.scm (imported-files): Rewrite to call to
'add-file-tree-to-store'.  Remove #:system and #:guile parameters.
(%imported-modules): Adjust docstring to say that result is a store
item, and adjust 'imported-files' call.
(%compiled-modules): Adjust accordingly.
(imported+compiled-modules): Likewise.
(build-expression->derivation): Likewise.