This implements the following defaulting rules:

 * For `cabal-version:2.0` and below, default to the `Custom`
   build-type unconditionally (legacy defaulting)

 * Otherwise, if a `custom-setup` stanza is defined, default to
   the `Custom` build-type; else default to `Simple` build-type.

This gets us better defaults for the two most popular use-cases, and
which can be statically inferred by only looking at the `.cabal` file.

This allows us to bring down the minimal (modern) trivial cabal
package definition down to a single file with 4 lines:

    cabal-version: 2.1
    name: mu
    version: 0
    library

NB: We don't need any `Setup.hs` file, as `cabal sdist` will magically
generate one on the fly.