Mark external package as extra-dep

[jameshfisher]

I don't fully understand why I needed to do this. But I was getting this build error:

$ stack build

Error: While constructing the build plan, the following exceptions were encountered:

In the dependencies for stronghold-0.1:
    zookeeper must match -any, but the stack configuration has no specified version
needed since stronghold is a build target.

Some potential ways to resolve this:

  * Set 'allow-newer: true' to ignore all version constraints and build anyway.

  * You may also want to try using the 'stack solver' command.

Plan construction failed.

Which led me to this recent stack issue: https://github.com/commercialhaskell/stack/issues/3728

[jameshfisher]

Aha, I see this is part of #10 !

[jameshfisher]

closing in favor of #10