`process-load-or-reload` suddenly requires `stack`

When I M-x process-load-or-reload today I started getting

```
apply: Searching for program: no such file or directory, stack
```

This seems to have regressed recently with f4b184181db3ac1ac3698b3b61702ab60f26808f (@purcell) which incorrectly assumes that the presence of a `stack.yml` file implies that the user wants to use `stack` even though it was maybe just added there for convenience to other Stack users.

It's perfectly fine if `haskell-mode` adds support for Stack, but please, don't impose the non-default tool `stack` on users in its default configuration. Even if I happen to have `stack` installed, I certainly don't want it to hijack the REPL mode in the `auto`-mode.

Start with the M-x process-load-or-reload entry point and inspect regression commit f4b184181db3ac1ac3698b3b61702ab60f26808f, especially how stack.yml selects the command. Reproduce with a project containing stack.yml but no stack executable. Done means the default auto mode no longer requires or invokes stack solely because stack.yml is present.

Written by the indexing model from the issue text.