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
This support was actually added by removing a superfluous check that was
too eager in cmLinkItemGraphVisitor.cxx.
Extended the existing Graphviz tests to include this particular case,
and re-generated the expected output files.
Fixes: #20720
* Added a fairly comprehensive test suite
* Separated the graph traversal logic from the Graphviz generation
code by introducing a new class, cmLinkItemsGraphVisitor{.h,cxx}
* Made the graph traversal logic less ad-hoc by using existing
methods in the GlobalGenerator; this fixed a few bugs
* Added support for new target types: custom targets, object
and unknown libraries
* Improved support for ALIAS libraries by showing the alias(es)
in the graph
* Introduced new flags to control those new libraries (consistent
with existing flags)
* Updated the documentation
* Removed useless setting to set graph type in dot file
* Improved the node/edge shapes (nicer, more consistent)
* Added a legend to the graph
* Some refactoring and cleanup of the Graphviz generation code
* Added test and fix for issue 19746
