Add a test for when source-dirs is a list.

[philderbeast]

Please accept a list of source directories.

[philderbeast]

I have built this test using stack and using cabal and note that both needed base to be explicitly listed in the dependencies. If base was not listed then they errored;

source-dirs> stack ghci
...
<interactive>:1:19: error:
    Not in scope: ‘System.IO.hSetBuffering’
    No module named ‘System.IO’ is imported.
...

source-dirs> nix-shell -p haskell.compiler.ghc822 -p haskell.packages.ghc822.cabal-install
[nix-shell:]$ cabal new-repl
...
<interactive>:1:19: error:
    Not in scope: ‘System.IO.hSetBuffering’
    No module named ‘System.IO’ is imported.
...