Currently, it is quite hard to override anything about dfhack (for
example, to add plugins at build time), because
1. it is not exposed anywhere, so you have to do something like
`dwarf-fortress.override (oldArgs: { dfhack = oldArgs.dfhack.override...; })`
2. the final `dfhack` derivation is a `buildEnv`, so `overrideAttrs`
doesn't work as expected.
This fixes 1 by adding `dfhack` to the wrapper's `passthru`, and 2 by
moving `twbt.lib` (which is the only reason for the dfhack `buildEnv`)
into the wrapper.
Also adds an `extraPackages` argument to the wrapper, and copies `*.init`
files from the env to the DF directory so that we can easily add a
`dfhack.init`.
With these changes, I was able to build dfhack with the dfplex plugin.
839fdd101c