3f1df5f6

By: Tom Sydney Kerckhove <syd@cs-syd.eu>

Fix `ignore` filter leaking through `$` and ExpandedThingTc

Two bugs prevented the plugin's `ignore` config from working in practice:

1. `nix/addManifest.nix` passed the YAML config via `--ghc-options=...`
   (plural).  Cabal treats that as a space-separated multi-flag and the
   value's embedded `=` and `:` confused the parser, so the plugin was
   invoked with no opts and `ignore` always stayed at `[]`.  Switch to
   `--ghc-option=...` (singular) so the whole `-fplugin-opt=...` token
   reaches GHC intact.

2. Once the config is actually loaded, `opOccName` still failed to see
   through GHC 9.6+ expansions: `mark $ unwords [...]` is a
   `XExpr (ExpandedThingTc _ (HsApp (HsApp ($) mark) (unwords [...])))`,
   and `opOccName` returned `Nothing` for it.  The ignore filter then
   fell through to children and mutated the inner list literal.  Walk
   through `ExpandedThingTc`, and treat `($) f x` as having syntactic
   head `f` rather than `$`.

Regression test in `sydtest-mutation-example/src/Example/IgnoreLib.hs`:
`mark` is a two-argument no-op with a polymorphic payload (introduces
typecheck `WrapExpr` wrappers), called both directly and via `$`; the
golden `Example.IgnoreLib.json` asserts no mutations are produced.
`mutationManifestCheck.nix` now passes `./mutation.yaml` so the golden
actually exercises the `ignore` machinery.

Timeline