Add tests Paths_ autogen module + default.extensions

Fixes #5086

The https://github.com/haskell/cabal/pull/5054 links to
https://github.com/commercialhaskell/stack/issues/3789 which says

- `Ensure you have OverloadedStrings and RebindableSyntax extensions
  enabled.`

So we warn only in that case. Only `OverloadeStrings` (or
`OverloadedLists`) or `RebindableSyntax` seems to be ok.

Also make `allBuildInfos` return all (not only buildable) build infos,
removing FIXME. `allBuildInfos` is used only in D.PD.Check.