Adding support for DDC's Library Bundle module system.

[Markzipan]

This module system - in conjunction with the Frontend Server - supports web hot reload. This is part of an effort to deprecate DDC's older module systems.

Notable updates:

• Plumbing a ddc-library-bundle flag through several builders.
• Copied/modified DDC tests
• Updated bootstrapper logic for the new module system.
• Updated some conflations between webHotReload and the new module system. These should be consistent now.

[github-actions]

Package publishing

Package	Version	Status	Publish tag (post-merge)
package:build	4.0.3	already published at pub.dev
package:build_config	1.2.0	already published at pub.dev
package:build_daemon	4.1.1	already published at pub.dev
package:build_modules	5.1.5	ready to publish	build_modules-v5.1.5
package:build_runner	2.11.0-wip	WIP (no publish necessary)
package:build_test	3.5.5-wip	WIP (no publish necessary)
package:build_web_compilers	4.4.6	(error) pubspec version (4.4.6) and changelog (4.4.7) don't agree
package:scratch_space	1.2.0	already published at pub.dev

Documentation at https://github.com/dart-lang/ecosystem/wiki/Publishing-automation.

[github-actions]

PR Health

Changelog Entry ✔️

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[davidmorgan]

Looks good, thanks!

[bkonyi]

LGTM!