RFE: `which` command for build-tool-depends

I would like to be able to find an executable from the text editor that I can then call

https://gitlab.com/tseenshe/haskell-tng.el/blob/4e2cf006eb8c544ace8aafd2e8375dcb31dd8c8e/haskell-tng-hsinspect.el#L130-143

However, using `cabal exec -v0 which -- TOOL` fails to succeed until the project has been compiled because `cabal` does not know what to do.

Note that the current failure message is

```
dist-newstyle/tmp/environment.-63855: createDirectory: does not exist (No such file or directory)
```

which is not very helpful.

This would be a competitive advantage over `stack`.