Fix component registering in subdirectories

[pum1k]

Description

This PR allows using rclcpp_components_register_node and rclcpp_components_register_nodes functions in subdirectories added by add_subdirectory (and other non global scopes). This was done by storing the internal data in a property of the project directory instead of variable.

Fixes: #1698

Is this user-facing behavior change?

Yes, users will be able to use the above mentioned functions inside nested scopes.

Did you use Generative AI?

No

[fujitatomoya]

lgtm with green CI.

i think that the approach of using directory properties on PROJECT_SOURCE_DIR is the right fix to avoid the CMake variable scoping problem cleanly without polluting the cache.

one minor suggestion: would it be worth adding a brief comment in the hook file explaining why properties are used like original problem?

[pum1k]

@fujitatomoya Is this what you meant?