Use original YAML to preserve top-level order.

[philderbeast]

Sadly, we do not have a format-and-comment-preserving YAML parser/writer available that I'm aware of. There are other cases where Stack has to modify the the stack.yaml with the same problem. It's really annoying.
#3136 (comment)

Please include the following checklist in your pull request:

• Any changes that could be relevant to users have been recorded in ChangeLog.md.
• The documentation has been updated, if necessary

I tested with the stack.yaml file in this repo. Want to get this reviewed before adding tests. The commands I used for testing were:

> cabal run stack -- config set system-ghc true
> cabal run stack -- config set system-ghc false
> cabal run stack -- config set install-ghc true
> cabal run stack -- config set install-ghc false
> cabal run stack -- config set resolver lts-19.17
> cabal run stack -- config set resolver nightly

I added blanks lines at the start and end of the YAML document and these are preserved. I also moved the appended item to somewhere in the middle of the document.

[mpilgrem]

Thanks! Forgive me if it takes me a little while to work through this. My immediate priority is elsewhere (I'm currently trying to get Stack to build with GHC 9.2.4).

[philderbeast]

For testing I copied stack.yaml to stack__.yaml and used a difftool to compare. This is the starting state of both files with blank lines at the start and end of the file and with system-ghc: true inserted already. The rest of the content is straight from the YAML content of the current stack.yaml in this repo:

system-ghc: true

resolver: lts-17.15

packages:
- .

extra-deps:
- hpack-0.35.0@rev:0
- pantry-0.5.6@rev:0 # https://github.com/commercialhaskell/pantry/pull/53
- rio-0.1.21.0@rev:0


docker:
  enable: false
  # repo: fpco/alpine-haskell-stack:8.10.4
  repo: fpco/alpine-haskell-stack@sha256:1024fe4b3b082a8df64d00e8563b3151220ed90af09604a8f7e1d44040500c30

nix:
  # --nix on the command-line to enable.
  packages:
  - zlib
  - unzip

flags:
  stack:
    developer-mode: true

ghc-options:
  "$locals": -fhide-source-paths

drop-packages:
# See https://github.com/commercialhaskell/stack/pull/4712
- cabal-install

Followed up by these 3 commands:

> cabal run stack -- config set resolver lts-19.17
> cabal run stack -- config set install-ghc false
> cabal run stack -- config set system-ghc false

After the fix:

Before the fix:

[mpilgrem]

Thanks! If you are interested in the stack config set command, here is another bug that may interest you: resolver and snapshot are synonyms in project-level configuration files, but stack config set resolver does not respect the existence of a snapshot key.