Adds support for "symbolic" components, which represent feature-level
capabilities of a package that do not correspond to actual build targets.
These are modeled as pseudo-targets, using the INTERFACE type as a base,
and can be queried via:
get_target_property(... <tgt> "SYMBOLIC")
This enables consumers to declare requirements on optional features
(e.g., SSL support) even when they do not map to concrete targets.
Fixes: #27187
