Summary:
Link plugin shared libraries to the "plugins/node_modules" dir so they can be used as peer dependencies by both public and fb-internal plugins.
```
plugins
- node_modules
-- fb_shared_lib (symlink pointing to "plugins/fb/fb_shared_lib")
-- public_shared_lib (symlink pointing to "plugins/public/public_shared_lib")
- fb
-- fb_shared_lib
-- fb_plugin (can now use both fb_shared_lib and public_shared_lib as peer dependencies)
- public
-- public_shared_lib
-- public_plugin (can now use both public_shared_lib and fb_shared_lib (optionally if it's an fb-internal repo) as peer dependencies)
```
Reviewed By: passy
Differential Revision: D27034936
fbshipit-source-id: 68ee5312060ff57649ae061dd7dd14f8beb6d4a1