Preserve order of `extra-source-files` from existing `.cabal` file

[webdevred]

Problem:
hpack reorders files in the extra-source-files section, which affects diff readability. Related: #617

Expected behavior:
Keep the original order exactly as written.

Example:

-- Before (Gild)

extra-source-files:
  README.md
  examples/jbeam/fender.jbeam
  examples/jbeam/frame.jbeam
  examples/jbeam/suspension.jbeam

-- After (hpack)

extra-source-files:
  README.md
  JBFL_DOCS.md
  examples/README.md
  examples/jbeam/fender.jbeam
  examples/jbeam/frame.jbeam
  examples/jbeam/suspension.jbeam