qpkg: added parsing and recording of Provides data

- pkg.h (enum flags): new flags QPKG_PROVIDED to indicate that the
  package is only a provision, not an installable package
- fixup.h, fixup.c (complete_provisions): search for provisions that do
  not have a real package associated with them, create a package for
  them, and mark it as QPKG_PROVIDED
- qpkg.c (do_fixups): invoke complete_provisions
- pkg.h (struct pkg): add list of (virtual or real) packages a given
  package provides
- pkg.c (new_pkg): initialize pkg->provides
- pkg.c (free_pkg): deallocate pkg->provides
- gobble.c (gobble_buf): parse the list of packages a package provides
- TODO: updated status of Provides support

qpkg/TODO

@@ -6,6 +6,9 @@ Still left to do
 
 - handle Provides:
 
+  Update: Provides data is now parsed and properly integrated in the
+  package database, but not yet used to resolve prerequisites.
+
 - sort prerequisites such that they can be installed in the specified order
 
 - consider Architecture: