Require top-level `build-depends` when it occurs in all branches of a conditional

This is the companion issue of https://github.com/haskell/cabal/issues/7768.

See e.g. https://github.com/jvranish/Lenses/issues/3.
Currently hackage does not allow revision of conditionals, and it does not allow new `build-depends` in a revision.
In the situation (1)
```
build-depends: base
, A
if cond
build-depends: B >= 0.1 && < 0.4
else
build-depends: B >= 0.2 && < 0.5
```
the constraints for `B` cannot be revised.
Thus, the better practice is to put a top-level `B` with extra constraints in the conditional (2):
```
build-depends: base
, A
, B >= 0.1 && < 0.5
if cond
build-depends: B < 0.4
else
build-depends: B >= 0.2
```
hackage uploads could be required to add a top-level `build-depends` if it appears in all branches. So (1) would be rejected, and a suggestion of adding `B` to the top-level dependencies could be given:
```
build-depends: base
, A
, B
if cond
build-depends: B >= 0.1 && < 0.4
else
build-depends: B >= 0.2 && < 0.5
```
Enforcing this discipline would help with revisions on hackage that tighten constraints.