Reorganize `src/` directory (#87)

Test Plan:
Note that tests still pass, and all changes to snapshot files are
verbatim moves.

wchargin-branch: reorg

config/paths.js

@@ -60,7 +60,7 @@ module.exports = {
   // point within the build directory.
   backendEntryPoints: {
     fetchAndPrintGitHubRepo: resolveApp(
-      "src/backend/bin/fetchAndPrintGitHubRepo.js"
+      "src/plugins/github/bin/fetchAndPrintGitHubRepo.js"
     ),
   },
 };

src/plugins/github/fetchGitHubRepoTest.sh

@@ -19,7 +19,7 @@ main() {
     >"${output}" \
     ;
   diff -uw \
-    src/backend/githubDemoData/example-repo.json \
+    src/plugins/github/demoData/example-repo.json \
     "${output}" \
     ;
   rm "${output}"

src/plugins/github/githubPlugin.js

@@ -1,8 +1,8 @@
 // @flow
 
-import type {Node, Edge} from "./graph";
+import type {Node, Edge} from "../../core/graph";
-import type {Address} from "./address";
+import type {Address} from "../../core/address";
-import {Graph} from "./graph";
+import {Graph} from "../../core/graph";
 const stringify = require("json-stable-stringify");
 
 export const GITHUB_PLUGIN_NAME = "sourcecred/github-beta";

src/plugins/github/githubPlugin.test.js

@@ -1,7 +1,7 @@
 // @flow
 
 import {GithubParser, getNodeType, getEdgeType} from "./githubPlugin";
-import exampleRepoData from "./githubDemoData/example-repo.json";
+import exampleRepoData from "./demoData/example-repo.json";
 
 describe("GithubParser", () => {
   describe("whole repo parsing", () => {