Fix module_builder reading DDC modules for non-primary dart libraries.

[Markzipan]

Fixes #4271

DDC (old module system) names .module files on the primary input of their module cluster. WIthout this guard, we were looking for this file with non-primary libraries (hence file not found errors). Thanks for investigating @davidmorgan !

[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	5.1.4	ready to publish	build_modules-v5.1.4
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	4.4.3	already published at pub.dev
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 ✔️

Package	Changed Files

Changes to files need to be accounted for in their respective changelogs.

This check can be disabled by tagging the PR with skip-changelog-check.