Provide the location of parsed files in metadata

Rather than hardcoding parsed file lookup into `expandmetadata.xslt`
and `catalog2transform.xslt`, provide a file U·R·I for the parsed file
as a part of its metadata and use that instead. It _is_ reasonable for
transforms to want to access the original parsed documents of
dependencies. Note, however, that there is no guarantee that the parsed
document actually exists if it _isn’t_ a dependency for the file.