Configurable outputs for the combining builder

[simolus3]

This adds support for custom output locations, including outputs in different directories, to the combining builder.

• Make output extensions configurable through builder options. The options are validated to ensure the combining builder is running on Dart files and emits Dart files
• Move the check ensuring we have a part statement from the SharedPartBuilder to the CombiningBuilder. The former can't know the correct output location anymore.

This is related to #272, where the implementation in build requires cooperation from builders. Given that most generated files come from shared part generators in practice, this would enable generating to different folders for most users.

[jakemac53]

I filed dart-lang/sdk#46939 for the analyzer failure, we may need to mark that test as skipped for now.

Or possibly we should make the test smart enough to not complain about these warnings? It will break for every new minor sdk version.

[simolus3]

It works with the newest analyzer, I've fixed deprecation warnings which requires using the latest version as a lower bound.

I agree that we should eventually make the tests more reliable, I also had some of them failing when running in isolation because of the "Generating SDK summary" logs.