Fix provides() when passed a list of files

Fix provides() when passed a list of files

provides(files => [ ... ]) never worked before, because
package_versions_from_directory() must always be passed a directory name, for
use in abs2rel when resolving found files.

Plus the ability to combine the 'files' and 'prefix' options has been removed,
as it is paradoxical anyway - we cannot know what bits of the path would need
to be swapped out for the prefix.