Stack does work before needed

I just tried opening a package's .cabal file. I wasn't going to work on it (or at least not on the Haskell source, more for documentary fixups) but it seemed to take a while and hang Emacs before I could eventually get `C-g` to seemingly do anything.

After I had checked the .cabal file and did some stuff in non-Haskell source files, I went to go and do a git commit to find a `.stack-work` directory. Since I had just cloned this I know it wasn't there before, so it seems that just opening the `.cabal` file caused this.

Start by reproducing the report in Emacs: open a package's .cabal file in a freshly cloned project, then check whether .stack-work appears. Trace the handling of .cabal buffers and compare it with non-Haskell files. Done means opening the file no longer performs unnecessary Stack work or creates .stack-work before it is needed.

Written by the indexing model from the issue text.