Cabal does not fail on non-existing system build-tools

[sol]

Since Cabal >= 2.0 it is possible to list arbitrary executables as build-tools. According to the documentation those executables need to be on the PATH. However, cabal will silently ignore non-existing executables.

hspec/hspec-example@512125d demonstrates the issue.

Steps to reproduce:

$ cabal --version
cabal-install version 2.2.0.0
compiled using version 2.2.0.1 of the Cabal library
$ git clone -b build-tools-bug https://github.com/hspec/hspec-example/
$ cd hspec-example
$ cabal new-test

Expected behavior: cabal complains about the missing build tool

Actual behavior: cabal runs the test suite

[hvr]

cc'ing @Ericson2314 who might be familiar with the issue

[Ericson2314]

We should have soem sort explicit "foreign/system build tool depends" thing to desugar unknown build-tools into. That is the last step to build-tools being 100% desugared and deprecated. The fact that it is so close but not quite is probably the source of this bug.