Also add symlinks from the old `bin/` and `tests/` locations to avoid
breaking scripts and other tools.
Motivations:
* Scripts and tests no longer have to do dubious things like:
require('ep_etherpad-lite/node_modules/foo')
to access packages installed as dependencies in
`src/package.json`.
* Plugins can access the backend test helper library in a non-hacky
way:
require('ep_etherpad-lite/tests/backend/common')
* We can delete the top-level `package.json` without breaking our
ability to lint the files in `bin/` and `tests/`.
Deleting the top-level `package.json` has downsides: It will cause
`npm` to print warnings whenever plugins are installed, npm will
no longer be able to enforce a plugin's peer dependency on
ep_etherpad-lite, and npm will keep deleting the
`node_modules/ep_etherpad-lite` symlink that points to `../src`.
But there are significant upsides to deleting the top-level
`package.json`: It will drastically speed up plugin installation
because `npm` doesn't have to recursively walk the dependencies in
`src/package.json`. Also, deleting the top-level `package.json`
avoids npm's horrible dependency hoisting behavior (where it moves
stuff from `src/node_modules/` to the top-level `node_modules/`
directory). Dependency hoisting causes numerous mysterious
problems such as silent failures in `npm outdated` and `npm
update`. Dependency hoisting also breaks plugins that do:
require('ep_etherpad-lite/node_modules/foo')
Rewrite the `callAll` and `aCallAll` functions to support all
reasonable hook behaviors and to report errors for unreasonable
behaviors (e.g., calling the callback twice).
Now a hook function like the following works as expected when invoked
by `aCallAll`:
```
exports.myHookFn = (hookName, context, cb) => {
cb('some value');
return;
};
```
