slot op: do not remove := from vdb on unmatched dependencies.

Previously, the slot operator processing code used to remove := operator
from vdb dependency entries when the underlying package was not
installed. With regards to a dependency alike || ( A:= B:= ), this
suggested that the particular slot/subslot of installed package could
be replaced by any slot/subslot of the other package. In particular, it
confused dependency comparison in @changed-deps.

Instead, store the original := (or :slot=) atom in vdb. It does not
trigger any special behavior in the current dependency resolver, yet
keeps the extra information. It also makes proper @changed-deps
comparisons possible since the comparison code can know if the original
ebuild had the slot operator.