[dart][dio] Respect import-mappings parameter

[kuhnroyal]

• first check import mappings before defaulting to model import
• should fix [dart-dio-next] [BUG] Import mappings are not working #10522

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[kuhnroyal]

CC @jaumard (2018/09) @josh-burton (2019/12) @amondnet (2019/12) @sbu-WBT (2020/12) @kuhnroyal (2020/12) @agilob (2020/12) @ahmednfwela (2021/08)

[ahmednfwela]

will this allow importing types from external libraries?

[ahmednfwela]

also we need a way to modify the generated pubspec.yaml based on the referenced libraries

[kuhnroyal]

It does, but I think if you need a custom library in the pubspec, you should probably use a custom template for that file.