Help: use package-specific var for supported component list

Always using _supported_components causes conflicts when transitive dependencies also set this variable, e.g., because the developers followed these code samples.

An even more general approach could be to set _${CMAKE_FIND_PACKAGE_NAME}_supported_components instead, but such a change should then be applied to other variables for consistency.

Fixes: #23112 (closed)
Backport: release
Topic-rename: doc-example-package-components