LINK_LIBRARIES: Evaluate separately for linking and usage requirements

We evaluate `LINK_LIBRARIES` and `INTERFACE_LINK_LIBRARIES` for two purposes:

* Constructing the link line.
* Collecting usage requirements.

We evaluate `INTERFACE_LINK_LIBRARIES` separately for each purpose in
order to support the `$<LINK_ONLY:...>` generator expression used to
express private link dependencies of a static library.  Previously we
only evaluated `LINK_LIBRARIES` for linking, and used that result for
collecting usage requirements too.  Therefore `$<LINK_ONLY:...>` does
not work in `LINK_LIBRARIES`.

With the introduction of `INTERFACE_LINK_LIBRARIES_DIRECT`, evaluation
of `LINK_LIBRARIES` now needs to distinguish these two cases in order to
honor link dependencies encountered through `$<LINK_ONLY:...>` without
also exposing other usage requirements through private dependencies of a
static library.  Revise internal infrastructure to distinguish the two
cases when evaluating `LINK_LIBRARIES`.  Make the information available
in code paths for `INTERFACE_LINK_LIBRARIES_DIRECT` and `LINK_ONLY`.
Defer actually using the information to later commits.

Issue: #22496

Source/cmGeneratorExpressionNode.cxx

@@ -1415,7 +1415,8 @@ static std::string getLinkedTargetsContent(
 {
   std::string result;
   if (cmLinkImplementationLibraries const* impl =
-        target->GetLinkImplementationLibraries(context->Config)) {
+        target->GetLinkImplementationLibraries(
+          context->Config, cmGeneratorTarget::LinkInterfaceFor::Usage)) {
     for (cmLinkImplItem const& lib : impl->Libraries) {
       if (lib.Target) {
         // Pretend $<TARGET_PROPERTY:lib.Target,prop> appeared in our