Build SDKs for the DDC library bundle format

[srujzs]

In order to support hot reload, we initially planned to use the DDC module format. When that became infeasible, we introduced the library bundle format.

This format can only be emitted when passed as the single module arg to a DDC compile, so this change splits the generation of SDKs into multiple steps.

We also do not support unsound null safety with the new format, so it should never be generated.

When Flutter tools no longer tests for the DDC module format and removes support for it (which I'm tackling separately as well), the generated SDKs for that format can be deleted.

Part of the larger effort to support hot reload: dart-lang/webdev#2516

Pre-launch Checklist

• I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
• I read the [Tree Hygiene] wiki page, which explains my responsibilities.
• I read and followed the [Flutter Style Guide] and the [C++, Objective-C, Java style guides].
• I listed at least one issue that this PR fixes in the description above.
• I added new tests to check the change I am making or feature I am adding, or the PR is [test-exempt]. See [testing the engine] for instructions on writing and running engine tests.
• I updated/added relevant documentation (doc comments with ///).
• I signed the [CLA].
• All existing and new tests are passing.

[srujzs]

I suppose since we're going forward with the monorepo I'll move this change over there once that's finished. :)

[yjbanov]

This PR now needs to be migrated to the flutter/flutter monorepo.

[srujzs]

Yup! Now that we have a monorepo, I'm going to bundle this change in with the framework change that actually uses the artifacts, but that requires a version of DWDS to be published. I'll add you once I have that PR unblocked.