Auto-enable dependencies option

[dstoup]

It would be really nice if there was a Fletch option to auto-enable dependencies. If I'm building a custom subset of Fletch, I often want to say I need X, Y, and Z from Fletch, but also anything else that Fletch needs to build those things. There are many packages in Fletch that we just don't use directly, but we need to remember which things we need to also enable to get the things we care about.