[dart][dart-dio] Improve prefixing of inner enums with classname

• prevent further name conflicts by correctly naming the enum, until now there could potentially occur conflicts e.g. 2x MapTestInnerEnum by renaming the items child property
• correctly set enumName to match datatypeWithEnum

The inline enum template has been changed but only enums that were previously called InnerEnum have been changed. I don't think that was supported before all the recent changes so I don't think this is a breaking change.

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CC @swipesight (2018/09) @jaumard (2018/09) @josh-burton (2019/12) @amondnet (2019/12) @sbu-WBT (2020/12) @kuhnroyal (2020/12) @agilob (2020/12)