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

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 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
```
If `cabal check` would suggest a adding a top-level `build-depends` that appears in all branches, that would help with revisions on hackage.

Of course, this could also be fixed on the hackage side; in case it is fixed there by rejecting packages that could be rewritten from (1) to (2), `cabal check` should also warn about possible complaints by the hackage server.

Start with the cabal check behavior described in this issue and the two conditional build-depends examples. Determine how to detect a dependency present in every conditional branch, then make cabal check report a suggestion for a top-level dependency and verify that the warning covers the Hackage revision concern.

Written by the indexing model from the issue text.