`base` should be considered necessary when implicit prelude is used

[ryantrinkle]

Currently, packunused considers this package's use of base redundant; however, removing base from the cabal file results in a failing build, due to the implicit import of Prelude.

Although this minimal example may seem a bit contrived, the issue came up for me in a real codebase where an executable section was defined in which nothing from Prelude was used, since the executable was defined very straightforwardly in terms of the library.