Discover Cabal Packages should handle root accordingly

[LeanderK]

I was testing the Discover Cabal Packages action on a sample project that i have just imported (not via stack). It has the normal structure you get by calling stack new <name>

src/
ExampleProject.cabal
stack.yaml

now calling Discover Cabal Packages leads to declaring the some weird configuration where most of my files disappear (including the src/ directory).

[LeanderK]

Since i had to make some small modification to the Discover Cabal Packages action by working on #298 (and therefore the tests) i am already familiar with the code and can fix it.