`cabal.project` may reference cabal files instead of directories

[roberth]

Cabal install accepts references to .cabal files, as well as references to directories containing them. Haskell-flake does not support files yet.

Example:

cabal.project

packages:
  foo/foo.cabal

[srid]

Another way in which the parser is deficient is that if the cabal.project has only "packages" field and doesn't end in newline, the last package will not be recognized. cf. https://github.com/srid/ema/actions/runs/4410050701/jobs/7727093450 & https://github.com/srid/ema/commit/b46a08d7a26491b9801642ab9c13e94f929c6a90