[dart-dio] Use sourceFolder property instead of src

[zbarbuto]

The dart-dio generator had a couple of hard-coded /src/ paths which meant the resulting code didn't work if you used a the custom sourceFolder property as it was importing from a directory that didn't exist.

This change results in no differences in the generated code since the default source directory is already src (although a new line got added when I ran the scripts in the instructions so I've committed that) so should be non-breaking.

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[wing328]

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

[kuhnroyal]

LGTM, just need to remove that empty line.

[ahmednfwela]

it's fine , all of these files will get edited in my PR anyway after merging both dart generators

[zbarbuto]

Regenerated to fix all the extra new-lines.