haskell generic-builder: Fix package conf handling

Previously the package conf files were handled without paying attention
to the fact that it's pretty-printed output. One problem was discovered
with GHC 8.8.1 on Darwin, where the dynamic-library-dirs first field
seems to have increased in length, meaning while before it was

    dynamic-library-dirs: some-small-directory-name
                          some-more-directories

Now it is

    dynamic-library-dirs:
        some-larger-directory-name
        some-more-directories

Which breaks the code installed for https://github.com/NixOS/nixpkgs/pull/25537,
because that assumed the former format, resulting in the reoccurence of
the bug in https://github.com/NixOS/nixpkgs/issues/22810, see
https://github.com/Infinisil/all-hies/issues/43

This commit fixes this by "unprettyfying" the package conf files before
processing them.

Closes https://github.com/NixOS/nixpkgs/pull/78738.