Rethinking component management: backend as always-present exclusive base layer

[PHILO-HE]

#11009 (review)

Maybe it's uncertain. Let's keep track of this. @zhztheplayer

copied here:

This improvement looks nice. Thanks.

I have some thoughts based on the superficial observation, not related to this PR's purpose. Please feel free to correct me :)

It seems that backend is a special case — all components directly or indirectly depend on it (either the Velox backend or the ClickHouse backend for now). In addition, backend itself does not depend on other components.

If these assumptions always hold (even for future development), perhaps we could consider excluding the backend from component management. Since only one specific backend is selected at build time and component definitions are tied to that backend and isolated from those of other backends, this simplification might make sense.

With that exclusion:

We can focus component management solely on the common components.
For a component that depends only on the backend, its dependency list can be left empty (given that the backend is always implicitly present).
For a component A that depends on component B, we simply define A’s dependency on B as usual.
This approach would allow component management to remain independent of backend logic (and possibly make backend no longer extend component). It could simplify maintenance and make the structure clearer for developers.

[zhztheplayer]

Thank you @PHILO-HE . Let's get back to this when time is ripe.