Maintain a list of virtual provisions of packages from the official
binary package repositories. The list can be updated using the aurblup
script, e.g. via a cronjob.

This allows for adding proper links to package dependencies: If an AUR
package depends on a package from the official repositories (or on a
name provided by a package from the official repositories), add a link
to the corresponding archweb package details page. If an AUR package
depends on another AUR package (or on a name provided by another AUR
package), add a link to the corresponding aurweb package details page.
Otherwise, just display the name and do not add a link at all.

Fixes FS#46549.

Signed-off-by: Lukas Fleischer <lfleischer@archlinux.org>