Fix #390 - Add [:src][:packages] and deprecate [:build][:packages]

To ensure backwards compatibility, we use a unary merge:

```ruby
WebBlocks.config[:src][:packages] | WebBlocks.config[:build][:packages]
```

The only strangeness that will occur here is that .delete on `WebBlocks.config[:build][:packages]` will no longer work. **This needs to be noted in the release notes**