Further RNPM integration

Summary:
This commit removes `rnpm` folder that we left during initial merge to keep the diff cleaner. The `core`, `link` and `install` have now the same directory structure as any other command to make development more natural for all of us.

From most notable differences:
1) the `src` folder is now gone. The new structure should make it easier for people to work with the stuff and also move us closer to 100% rnpm integration,
2) There's also no `package.json` present in any of the `rnpm` packages, since they are no longer standalone modules,
3) There's no `bugs.url` in link.js since main package.json of React doesn't specify it. Decided to hardcode it to facebook/react-native since it's really unlikely to change. If one would prefer to use pkg.bugs.url as before, a separate PR modifying package.json should be sent.
Closes https://github.com/facebook/react-native/pull/9509

Differential Revision: D3751115

fbshipit-source-id: 74ae8330f7634df0887ad676808f47eee4b8de85

local-cli/core/getCommands.js

@@ -0,0 +1,12 @@
+const path = require('path');
+const findPlugins = require('./findPlugins');
+const flatten = require('lodash').flatten;
+
+/**
+ * @return {Array} Array of commands
+ */
+module.exports = function getCommands() {
+  const appRoot = process.cwd();
+  const plugins = findPlugins([appRoot]).map(name => require(path.join(appRoot, 'node_modules', name)));
+  return flatten(plugins);
+};