Add a CombiningBuilder for .g.dart files and document how to use it

[natebosch]

We added runs_before and applies_builder to build config so it should be possible now to put everything together with one or more source_gen part builders cooperating to generate a single part file. We'll want a "CombiningBuilder" ("MergingBuilder"?) in this package to read in all the files generated by different part builders and write them to a single output.

• Fix duplicate headers - Fixes for CombiningBuilder #348
• Fix logging of missing part directive - Fixes for CombiningBuilder #348
• Document usage in readme
• Update build.yaml to properly expose CombiningBuilder
• BLOCKING Get build issue fixed related to running built-to-cache builder over transitive dependencies - Filter out the builders which "applies_builders" some builder which is "build_to: source" on non root packages build#1578
• Add a cleanup builder

After published...

• Add a working example to repo/example

[kevmoo]

@grouma – added some work items before we can close this. The first two are handled by my pending PR

[kevmoo]

How are we doing w/ the check boxes here @natebosch and @grouma ?

[kevmoo]

...still need to tweak build.yaml, right?