Fixes for Dart imports

[insertjokehere]

Fixes https://github.com/google/flatbuffers/issues/7609 Fixes https://github.com/google/flatbuffers/issues/8379

This PR builds on the work from https://github.com/google/flatbuffers/pull/8259 to:

• Add a simple test to catch regressions in this area
• Fix an issue where include chains (a includes b which includes c) are naively represented as a single import in Dart, which doesn't reflect how Dart handles imports. I've adjusted the Dart code generator to include an export statement to match import statements for files in the same namespace to ensure that include chains work in the generated code

[fawdlstty]

@CasperN @vaind Sorry to tag you out of the blue. This part of the modification is very important to me. Can you promote the merger of PR?

[vaind]

I'm not a maintainer so I don't have merge permissions

[fawdlstty]

@aardappel Sorry to tag you out of the blue. Can you promote the merger of PR?

[aardappel]

This was merged: https://github.com/google/flatbuffers/pull/8313

Can you rebase this one, seems there may be some conflicts.

[aardappel]

There also seem to be dart codegen changes to the generated files that are not included in this PR

[insertjokehere]

@aardappel thanks for the review - I've rebased and updated the generated bindings. Happy to squash this down before merge.

[aardappel]

There also seem to be dart codegen changes to the generated files that are not included in this PR

This still seems to be an issue

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