discovery: Remove dependency on (guix ui).

discovery: Remove dependency on (guix ui).

This reduces the closure of (guix discovery) from 28 to 8 modules.

* guix/discovery.scm (scheme-files): Use 'format' instead of 'warning'.
(scheme-modules): Add #:warn parameter.  Use it instead of
'warn-about-load-error'.
(fold-modules): Add #:warn and pass it to 'scheme-modules'.
(all-modules): Likewise.
* gnu/bootloader.scm (bootloader-modules): Pass #:warn to 'all-modules'.
* gnu/packages.scm (fold-packages): Likewise.
* gnu/services.scm (all-service-modules): Likewise.
* guix/upstream.scm (importer-modules): Likewise.