[TypeScript] Split composed types in new addImports() method

[sarumont]

6430aaf introduced additional paths to add imports when resolving a deeply-nested object. The override in AbstractTypeScriptClientCodegen which splits composed types is not always hit, so this causes imports looking like:

import {Foo & Bar} from '.';

The versions don't match up (this change was introduced in 5.4.0), but this is the same bug as #9163 describes, from what I can tell.

If necessary, I can open a different bug.

Also, I have made this PR against 5.4.x, merging changes from the v5.4.0 tag that weren't in that branch for whatever reason. It applies cleanly to master as well, so if no patch is planned for 5.4.x, I can rebase to master.

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@TiFu (2017/07) @taxpon (2017/07) @sebastianhaas (2017/07) @kenisteward (2017/07) @Vrolijkx (2017/09) @macjohnny (2018/01) @topce (2018/10) @akehir (2019/07) @petejohansonxo (2019/11) @amakhrov (2020/02)

[macjohnny]

thanks for your contribution!
can you add a test in e.g. modules/openapi-generator/src/test/java/org/openapitools/codegen/typescript/fetch/TypeScriptFetchModelTest.java?