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/testing.md

@@ -0,0 +1,17 @@
+# Testing
+
+## Complement
+
+Have a look at [Complement's repository][complement] for an explanation of what
+it is.
+
+To test against Complement, with Nix and direnv installed and set up, you can
+either:
+
+* Run `complement "$COMPLEMENT_SRC" ./path/to/logs.jsonl ./path/to/results.jsonl`
+  to build a Complement image, run the tests, and output the logs and results
+  to the specified paths
+* Run `nix build .#complement` from the root of the repository to just build a
+  Complement image
+
+[complement]: https://github.com/matrix-org/complement