Instead of storing just the string names in these structures, lookup any
target associated with each item and store its cmTarget pointer.  Use
the cmLinkItem class to hold the name and pointer together.  Update
client sites to use the pre-stored lookup result instead of looking up
the target name again.

Create a cmTarget::LookupLinkItems helper method to handle the lookup.
Since lookups are now moving from cmComputeLinkDepends::AddLinkEntries
to cmTarget::LookupLinkItems, move use of CheckCMP0004 to the latter.
This drops use of CheckCMP0004 from entries added for _LIB_DEPENDS
variables by cmComputeLinkDepends::AddVarLinkEntries, but I do not
think that use was intentional originally anyway.