sc2: add graph subcommand (sourcecred#1811)

Summary:
Paired with @decentralion.

Test Plan:
Follow the test plan for sourcecred#1810, then additionally run

```
(cd /tmp/test-instance && node "$OLDPWD/bin/sc2.js" graph)
```

and note that the `output/graphs/...` directory has a graph JSON file.

wchargin-branch: cli2-graph

src/cli2/graph.js

@@ -0,0 +1,55 @@
+// @flow
+
+import fs from "fs-extra";
+import sortBy from "lodash.sortby";
+import stringify from "json-stable-stringify";
+import {join as pathJoin} from "path";
+
+import {CascadingReferenceDetector} from "../core/references/cascadingReferenceDetector";
+import type {Command} from "./command";
+import {
+  makePluginDir,
+  loadInstanceConfig,
+  pluginDirectoryContext,
+} from "./common";
+import {toJSON as weightedGraphToJSON} from "../core/weightedGraph";
+
+function die(std, message) {
+  std.err("fatal: " + message);
+  return 1;
+}
+
+const graphCommand: Command = async (args, std) => {
+  if (args.length !== 0) {
+    die(std, "usage: sourcecred graph");
+  }
+  const baseDir = process.cwd();
+  const config = await loadInstanceConfig(baseDir);
+  const graphOutputPrefix = ["output", "graphs"];
+
+  const rd = await buildReferenceDetector(baseDir, config);
+  for (const [name, plugin] of config.bundledPlugins) {
+    const dirContext = pluginDirectoryContext(baseDir, name);
+    const graph = await plugin.graph(dirContext, rd);
+    const serializedGraph = stringify(weightedGraphToJSON(graph));
+    const outputDir = makePluginDir(baseDir, graphOutputPrefix, name);
+    const outputPath = pathJoin(outputDir, "graph.json");
+    await fs.writeFile(outputPath, serializedGraph);
+  }
+  return 0;
+};
+
+async function buildReferenceDetector(baseDir, config) {
+  const rds = [];
+  for (const [name, plugin] of sortBy(
+    [...config.bundledPlugins],
+    ([k, _]) => k
+  )) {
+    const dirContext = pluginDirectoryContext(baseDir, name);
+    const rd = await plugin.referenceDetector(dirContext);
+    rds.push(rd);
+  }
+  return new CascadingReferenceDetector(rds);
+}
+
+export default graphCommand;

src/cli2/sourcecred.js

@@ -3,6 +3,7 @@
 import type {Command} from "./command";
 
 import load from "./load";
+import graph from "./graph";
 
 const sourcecred: Command = async (args, std) => {
   if (args.length === 0) {
@@ -12,6 +13,8 @@ const sourcecred: Command = async (args, std) => {
   switch (args[0]) {
     case "load":
       return load(args.slice(1), std);
+    case "graph":
+      return graph(args.slice(1), std);
     default:
       std.err("fatal: unknown command: " + JSON.stringify(args[0]));
       return 1;