In multi-config generators we memoize the computed set of source files
for a target to avoid repeating the computation when the set does not
depend on the configuration.  We already track whether generator
expressions in `SOURCES` or `INTERFACE_SOURCES` reference the
configuration (`$<CONFIG:...>`).  However, we previously forgot to track
whether the set of libraries whose `INTERFACE_SOURCES` are considered
depends on the configuration.  This caused multi-config generators to
use the first configuration's set of sources for all configurations
in cases such as

    target_link_libraries(tgt PRIVATE $<$<CONFIG:Debug>:iface_debug>)

where the `iface_debug` target has `INTERFACE_SOURCES`.

Fix this by also tracking config-dependence of the list of libraries for
evaluation of the list of source files.

Fixes: #20683