Allow 'if' for data-files section

In the MicroHs cabal file I'd like to say this
```
data-files:
src/runtime/*.c
src/runtime/*.h
if impl(ghc)
data-files:
lib/**/*.hs
lib/**/*.hs-boot
```

But when I tried it, it didn't work. It works in MicroCabal so I've managed to hack around it, but it seems like an obvious thing that should work.

The issue names no repository file or test; start by locating the Cabal handling for data-files and conditional if impl(ghc) sections. Compare the behavior with MicroCabal, then add support for the shown conditional data-files form and verify it with an equivalent Cabal file or existing relevant tests.

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