`buildFromSdist` gets set to false when `defaults.settings.default` is present

[johnhampton]

#298 ensures that buildFromSdist is set to true for local packages. However if defaults.settings.default.haddock is set, buildFromSdist gets reset to false.

This results in a build with buildFromSdist set to true.

        haskellProjects.default = {
          # defaults.settings.default = { haddock = true;  };

          # Project settings here
        };

This results in a build with buildFromSdist set to false.

        haskellProjects.default = {
          defaults.settings.default = { haddock = true;  };

          # Project settings here
        };

I can force it back to true with:

        haskellProjects.default = {
          defaults.settings.default = { haddock = true; buildFromSdist = true; };

          # Project settings here
        };