Internally-imported packages are not making it into the package list

[sdboyer]

Once I saw it, this was a big, obvious 🤦‍♂️ .

Because the ExternalReach() function is focused entirely on external imports, it entirely omits any internal packages that may be a part of that import chain. As a result, we end up missing packages in deps that are only imported internally.

For example, if our root project imports a, and (only) a imports a/foo, then a/foo will not show up in the packages list.

The fix for this is fairly complicated. I'll have to roll it in as part of #148.