use lib.makeScope and files to organize packages

Some of the improvements here include:

* rocksdb can actually use jemalloc now instead of just pulling in a
  second rocksdb for no reason
* "complement-runtime" factored back out into shell file
* complement image no longer uses `mkDerivation` for `copyToRoot`
  because that's what `buildEnv` is for
* complement image no longer sets `SERVER_NAME`, complement already does
  that
* all packages were factored out into `callPackage`-able files for use
  with a custom `lib.makeScope pkgs.newScope`
* new version of `mkPackage` has options that are easier to use and
  override such as `features`

docs/development.md

@@ -0,0 +1,4 @@
+# Development
+
+Information about developing the project. If you are only interested in using
+it, you can safely ignore this section.