Add readAllSourcesFromFilesystem to resolveSources.

[davidmorgan]

For the benefit of source_gen_test which relies on the old behaviour https://github.com/davidmorgan/source_gen_test.

And, tweak error handling: something totally incomprehensible to do with zones and error handling was making one source_gen_test test fail despite the exception being caught.

[github-actions]

Package publishing

Package	Version	Status	Publish tag (post-merge)
package:build	2.5.4	already published at pub.dev
package:build_config	1.1.3-wip	WIP (no publish necessary)
package:build_daemon	4.0.5-wip	WIP (no publish necessary)
package:build_modules	5.0.14	already published at pub.dev
package:build_resolvers	2.5.4	already published at pub.dev
package:build_runner	2.5.4	already published at pub.dev
package:build_runner_core	9.1.2	already published at pub.dev
package:build_test	3.2.0	ready to publish	build_test-v3.2.0
package:build_web_compilers	4.1.6	already published at pub.dev
package:scratch_space	1.0.3-wip	WIP (no publish necessary)

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.