Add back in a reconfigure / rebuild optimization for allow-newer: true

[mgsloan]

The fix for #1579 requires us to pass all dependencies in to the configure step. We currently have a clever optimization that passes in --enable-tests or --enable-benchmarks if their dependencies are present. This reduces rebuilds due to changing cabal_macros.h files.

I spent some time trying to figure out how to move this decision to earlier in the build process. However, it got a bit head-scratching. I'll keep it in mind as I make more changes atop this.

For now, --exact-configuration is only enabled when allow-newer is set to true. The optimization is only disabled when allow-newer is set. Once this is fixed, I think --exact-configuration should always get passed.

[mgsloan]

Some comments I was going to commit, might as well put here instead:

                    -- 'Stack.Package.packageDependencies' needs to know
                    -- that we are going to enable tests / benchmarks.
                    -- This way, we can have the dependencies when
                    -- figuring out configuration options in
                    -- Stack.Types.Build.configureOpts, used by
                    -- "Stack.ConstructPlan". We should probably also
                    -- move the --enable-* arg generation to there too.