[ffigen] Config update part 2: imports

[liamappelbe]

• Consolidate Foo.imported and importedTypesByUsr into FfiGenerator.importType, using the callback pattern described in [ffigen] Dart API: Change Struct.imported and friends to a callback #2595
• Remove extension type Config utils
• Integers is now empty and can be deleted

Fixes #2596
Fixes #2595

[github-actions]

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Package	Version	Status	Publish tag (post-merge)
package:code_assets	1.3.0-wip	WIP (no publish necessary)
package:data_assets	0.20.0	already published at pub.dev
package:ffi	2.2.0	already published at pub.dev
package:hooks	2.2.0-wip	WIP (no publish necessary)
package:hooks_runner	1.6.2-wip	WIP (no publish necessary)
package:jni_flutter	1.0.2	already published at pub.dev
package:jni_util	1.0.0	already published at pub.dev
package:native_toolchain_c	0.19.4-wip	WIP (no publish necessary)
package:record_use	1.1.0	ready to publish	record_use-v1.1.0
package:swift2objc	0.3.0-wip	WIP (no publish necessary)
package:swiftgen	0.1.3	already published at pub.dev

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

[github-actions]

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[dcharkes]

Merci @liamappelbe ! 🙏