Multiple inclusion bugfix: only process rows with source

Required returns content only once, but since it's async it's not guaranteed that first request will get source, so we could only process rows with source.

Explanation by example:

1. require('some-module') in bunch of files
2. resulted file node:
```json
{ id: 23,
  source: '(function(process,global,__filename,__dirname){undefined\n})(require("__browserify_process"),window,"/node_modules/compound/lib/client/util.js","/node_modules/compound/lib/client")',
  deps: { child_process: 28, __browserify_process: 1 },
  entry: false }
```
note "undefined\n" as source.

it happens because of sync forEach processing of all rows, which is fine, but it rely's on order of files, and first row with '/node_modules/compound/lib/client/util.js' may have no source, because required() call only returns source once per id, all other times source=undefined.

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this is minimal changeset i've found to fix problem, probably you may like to come up with another way to fix it.