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

[kuhnroyal]

• 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

Follow-up for #8220

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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)

[kuhnroyal]

@wing328 Can you merge this please when you get a chance

[agilob]

Will you be ok with these changes being very temporary? I'm planning to use 3rd party library for enum unmarshalling and would like to introduce it soon for dart generator.

[kuhnroyal]

This is not really related. Even if you extract the enum serialization, you need the names of enums to be unique, which this change fixes. As the description says, you can still have name collisions without this fix. That would not change with a different form of serialization.